Created experimenter subtype for vendor's actions

ExperimenterActionSerializerKey has a new parameter which is type of vendor's action.
It allows to select right serializer directly in OFJava so a vendor does not need to care about it.
- created codec for Nicira match nxm_of_eth_type

Change-Id: Id9bc0f06820cab7b897eb710a70ddc3d6af13a45
Signed-off-by: Martin Sunal <msunal@cisco.com>