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.

pkg/fission-cli/flag/key/key.go

@@ -17,8 +17,9 @@ limitations under the License.
 package flagkey
 
 const (
-	Verbosity = "verbosity"
-	Server    = "server"
+	Verbosity  = "verbosity"
+	Server     = "server"
+	ClientOnly = "client-only"
 
 	resourceName = "name"
 	force        = "force"