bots: Modify 'virtual_fs' bot to respond with help message on bot mention.

Earlier, if virtual_fs bot is called without any argument, then the
bot returns nothing.
Now, virtual_fs bot replies with help message on how the user can
call this bot.

contrib_bots/bots/virtual_fs/virtual_fs.py

@@ -10,7 +10,7 @@ class VirtualFsHandler(object):
     def handle_message(self, message, client, state_handler):
         command = message['content']
         if command == "":
-            return
+            command = "help"
         sender = message['sender_email']
 
         state = state_handler.get_state()