Move Openflow NXM Extensions to its intended home: Openflowplugin

Move Openflow NXM Extensions to its intended home: Openflowplugin

This change stops referring to nxm extensions at the temporary location
and move it to where it was always meant to be.

Related gerrits:
https://git.opendaylight.org/gerrit/#/c/10428/
https://git.opendaylight.org/gerrit/#/c/13234/
https://git.opendaylight.org/gerrit/#/c/14289/
https://git.opendaylight.org/gerrit/#/c/14316/

Patch 2: Use constant for the version of openflow plugin feature
Patch 3: Use constant for the version of openflow plugin feature (cont.)

Change-Id: I6a1e2d3c41fee31e864fb8be415506c62518e099
Signed-off-by: Flavio Fernandes <ffernand@redhat.com>