Sanity: Will fail on the SNAPSHOT_METADATA_SERVICE plug in capability

[carlbraganza]

The csi.PluginCapability_Service_SNAPSHOT_METADATA_SERVICE capability should be recognized by the Identity service test in pkg/sanity/identity.go:

Describe("GetPluginCapabilities", func() {
		It("should return appropriate capabilities", func() {
		                ...
				By(fmt.Sprintf("[%s] checking successful response", name))
				for _, cap := range res.GetCapabilities() {
					switch cap.GetType().(type) {
					case *csi.PluginCapability_Service_:
						switch cap.GetService().GetType() {
						case csi.PluginCapability_Service_CONTROLLER_SERVICE:
						case csi.PluginCapability_Service_VOLUME_ACCESSIBILITY_CONSTRAINTS:
						case csi.PluginCapability_Service_GROUP_CONTROLLER_SERVICE:
						default:
							Fail(fmt.Sprintf("Unknown service: %v\n", cap.GetService().GetType()))
						}

I had overlooked this in #544.

[carlbraganza]

/assign @carlbraganza