Fix concurrency issues with helm install

Address a race condition that arises when running two `helm install`
commands, both of which specify a namespace that does not already exist.

In this specific instance, attempting to create a `namespace` which
already exists shouldn't be a failure which causes `helm install` to
terminate.

pkg/kube/namespace.go

@@ -40,7 +40,16 @@ func getNamespace(client internalclientset.Interface, namespace string) (*core.N
 func ensureNamespace(client internalclientset.Interface, namespace string) error {
 	_, err := getNamespace(client, namespace)
 	if err != nil && errors.IsNotFound(err) {
-		return createNamespace(client, namespace)
+		err = createNamespace(client, namespace)
+
+		// If multiple commands which run `ensureNamespace` are run in
+		// parallel, then protect against the race condition in which
+		// the namespace did not exist when `getNamespace` was executed,
+		// but did exist when `createNamespace` was executed. If that
+		// happens, we can just proceed as normal.
+		if errors.IsAlreadyExists(err) {
+			return nil
+		}
 	}
 	return err
 }

pkg/tiller/environment/environment.go

@@ -98,8 +98,6 @@ type Engine interface {
 type KubeClient interface {
 	// Create creates one or more resources.
 	//
-	// namespace must contain a valid existing namespace.
-	//
 	// reader must contain a YAML stream (one or more YAML documents separated
 	// by "\n---\n").
 	Create(namespace string, reader io.Reader, timeout int64, shouldWait bool) error