Augment OpenFlow NXM (vendor) extensions through proper extensibility framework

Openstack integration with OpenDaylight using OVSDB needs a few NXM vendor
extensions in place.

Yang Augments from external bundle/application is the correct approach in
getting this done. But unfortunately, the extensibility support is lacking
in the ofjava and ofplugin bundles.

In order to get the integration moving towards the Helium Release, we have
implemented the vendor extension handling inline to the ofplugin and ofjava bundles. This includes :
1. translation in ofplugin from generic to OF specifics.
2. serialization and deserialization for these vendor extensions in ofjava

https://git.opendaylight.org/gerrit/#/c/7633/
https://git.opendaylight.org/gerrit/#/c/7607/

The above gerrits are merged by the ofjava/ofplugin team as a temporary
measure to unblock the progress and until the extensibility support is
made available.

This bug is opened to track the vendor extension inline changes (more to come) in ofjava and ofplugin and revert them from these projects once the extensibility support is made available end to end.