provisioning: Refactor volume plugins.

NewPersistentVolumeTemplate() and Provision() are merged into one call.

pkg/volume/host_path/host_path_test.go

@@ -163,7 +163,7 @@ func TestProvisioner(t *testing.T) {
 	if err != nil {
 		t.Errorf("Failed to make a new Provisioner: %v", err)
 	}
-	pv, err := creater.NewPersistentVolumeTemplate()
+	pv, err := creater.Provision()
 	if err != nil {
 		t.Errorf("Unexpected error creating volume: %v", err)
 	}