Basic GroupMod support

Following changes are made to add GroupMod support:

1) Added correct values bucket type values according to OpenFlow1.3 specification.
2) Added command values to GroupMod.
3) Added 'id' parameter required by rfproxy for sending message to switch.
4) Corrected output string for GroupMod message.
5) Added GroupMod support to RFProtocol factory.

Signed-off-by: Srijan Mishra <srijan23mishra@gmail.com>