feat: tiller probing endpoints can be disabled

This adds the `--probe=[true|false]` flag to `tiller`, so that you can selectively disable the following probing HTTP endpoints:

- `/readiness`
- `/liveness`
- `/metrics`

One of expected use-cases of this feature would be to avoid consuming an extra port per `tiller`, which becomes more problematic in the [tillerless](https://github.com/rimusz/helm-tiller) setup.

The default is `--probe=true`, which starts the probing endpoints as before.

Implementation-wise, I intentionally made it so that the number of changed lines is as small as possible.
That is, I opted not to factor out the probes server starting logic into its own function, like `startProbesServer`.
Instead, I just added conditionals to the logging part and the server starting part.

As it isn't easily E2E testable, I've verified it to work by running the following commands manually.

With probing enabled(default):

```
$ ./tiller
[main] 2019/04/06 09:20:15 Starting Tiller v2.12+unreleased (tls=false)
[main] 2019/04/06 09:20:15 GRPC listening on :44134
[main] 2019/04/06 09:20:15 Probes listening on :44135
[main] 2019/04/06 09:20:15 Storage driver is ConfigMap
[main] 2019/04/06 09:20:15 Max history per release is 0
```

With probing disabled, you'll see no tiller is no longer listening on 44135:

```
$ ./tiller --probe=false
[main] 2019/04/06 09:20:07 Starting Tiller v2.12+unreleased (tls=false)
[main] 2019/04/06 09:20:07 GRPC listening on :44134
[main] 2019/04/06 09:20:07 Storage driver is ConfigMap
[main] 2019/04/06 09:20:07 Max history per release is 0
```

To ensure that tiller can disable the probing endpoints, I ran multiple tillers at once, with/without `--probe=false`:

The first test runs three tillers without `--probe=false`.

As expected, it results in two tillers failes due to the conflicting port, as you can see in the message `Probes server died: listen tcp :44135: bind: address already in use`.

```
$ bash -c 'for i in {0..2}; do (./tiller --listen=:$((44136+$i)) 2>&1 | sed "s/^/tiller $i: /" )& done; sleep 3 ; pkill tiller'
tiller 1: [main] 2019/04/06 09:57:49 Starting Tiller v2.12+unreleased (tls=false)
tiller 1: [main] 2019/04/06 09:57:49 GRPC listening on :44137
tiller 1: [main] 2019/04/06 09:57:49 Probes listening on :44135
tiller 1: [main] 2019/04/06 09:57:49 Storage driver is ConfigMap
tiller 1: [main] 2019/04/06 09:57:49 Max history per release is 0
tiller 0: [main] 2019/04/06 09:57:49 Starting Tiller v2.12+unreleased (tls=false)
tiller 0: [main] 2019/04/06 09:57:49 GRPC listening on :44136
tiller 0: [main] 2019/04/06 09:57:49 Probes listening on :44135
tiller 0: [main] 2019/04/06 09:57:49 Storage driver is ConfigMap
tiller 0: [main] 2019/04/06 09:57:49 Max history per release is 0
tiller 0: [main] 2019/04/06 09:57:49 Probes server died: listen tcp :44135: bind: address already in use
tiller 2: [main] 2019/04/06 09:57:49 Starting Tiller v2.12+unreleased (tls=false)
tiller 2: [main] 2019/04/06 09:57:49 GRPC listening on :44138
tiller 2: [main] 2019/04/06 09:57:49 Probes listening on :44135
tiller 2: [main] 2019/04/06 09:57:49 Storage driver is ConfigMap
tiller 2: [main] 2019/04/06 09:57:49 Max history per release is 0
tiller 2: [main] 2019/04/06 09:57:49 Probes server died: listen tcp :44135: bind: address already in use
```

The second test runs three tillers with `--probe=false`.

It results in all tillers running without errors, that indicates this feature is working as expected:

```
$ bash -c 'for i in {0..2}; do (./tiller --listen=:$((44136+$i)) --probe=false 2>&1 | sed "s/^/tiller $i: /" )& done; sleep 3 ; pkill tiller'
tiller 1: [main] 2019/04/06 09:58:18 Starting Tiller v2.12+unreleased (tls=false)
tiller 1: [main] 2019/04/06 09:58:18 GRPC listening on :44137
tiller 1: [main] 2019/04/06 09:58:18 Storage driver is ConfigMap
tiller 1: [main] 2019/04/06 09:58:18 Max history per release is 0
tiller 2: [main] 2019/04/06 09:58:18 Starting Tiller v2.12+unreleased (tls=false)
tiller 2: [main] 2019/04/06 09:58:18 GRPC listening on :44138
tiller 2: [main] 2019/04/06 09:58:18 Storage driver is ConfigMap
tiller 2: [main] 2019/04/06 09:58:18 Max history per release is 0
tiller 0: [main] 2019/04/06 09:58:18 Starting Tiller v2.12+unreleased (tls=false)
tiller 0: [main] 2019/04/06 09:58:18 GRPC listening on :44136
tiller 0: [main] 2019/04/06 09:58:18 Storage driver is ConfigMap
tiller 0: [main] 2019/04/06 09:58:18 Max history per release is 0
```

Resolves #3159

Signed-off-by: Yusuke KUOKA <ykuoka@gmail.com>

cmd/tiller/tiller.go

@@ -76,6 +76,7 @@ const (
 var (
 	grpcAddr             = flag.String("listen", ":44134", "address:port to listen on")
 	probeAddr            = flag.String("probe-listen", ":44135", "address:port to listen on for probes")
+	enableProbing        = flag.Bool("probe", true, "enable probing over http")
 	enableTracing        = flag.Bool("trace", false, "enable rpc tracing")
 	store                = flag.String("storage", storageConfigMap, "storage driver to use. One of 'configmap', 'memory', or 'secret'")
 	remoteReleaseModules = flag.Bool("experimental-release", false, "enable experimental release modules")
@@ -187,7 +188,9 @@ func start() {
 
 	logger.Printf("Starting Tiller %s (tls=%t)", version.GetVersion(), *tlsEnable || *tlsVerify)
 	logger.Printf("GRPC listening on %s", *grpcAddr)
-	logger.Printf("Probes listening on %s", *probeAddr)
+	if *enableProbing {
+		logger.Printf("Probes listening on %s", *probeAddr)
+	}
 	logger.Printf("Storage driver is %s", env.Releases.Name())
 	logger.Printf("Max history per release is %d", *maxHistory)
 
@@ -207,6 +210,10 @@ func start() {
 	}()
 
 	go func() {
+		if !*enableProbing {
+			return
+		}
+
 		mux := newProbesMux()
 
 		// Register gRPC server to prometheus to initialized matrix