Add fake client for local command operation (#1515)

This PR adds a fake controller client for local CLI operations like 
offline spec generation or for unit test purposes. The fake client 
now only implements the "Version()" function and more functions 
will be implemented once we figure out how to achieve the goals 
mentioned above.

cmd/fission-cli/app/app.go

@@ -44,13 +44,19 @@ func App() *cobra.Command {
 		PersistentPreRunE: wrapper.Wrapper(
 			func(input cli.Input) error {
 				console.Verbosity = input.Int(flagkey.Verbosity)
-				serverUrl, err := util.GetServerURL(input)
-				if err != nil {
-					return err
+
+				if input.IsSet(flagkey.ClientOnly) {
+					// TODO: use fake rest client for offline spec generation
+					cmd.SetClientset(client.MakeFakeClientset(nil))
+				} else {
+					serverUrl, err := util.GetServerURL(input)
+					if err != nil {
+						return err
+					}
+					restClient := rest.NewRESTClient(serverUrl)
+					cmd.SetClientset(client.MakeClientset(restClient))
 				}
-				restClient := rest.NewRESTClient(serverUrl)
-				// TODO: use fake rest client for offline spec generation
-				cmd.SetClientset(client.MakeClientset(restClient))
+
 				return nil
 			},
 		),