Use NXMs defined in openflowplugin.

Use NXMs defined in openflowplugin.

Group Based Policy was using Nicira extensions defined in
the OVSDB project. These extensions have migrated to the
OpenFlow plugin. This patch removes the reference to the
OVSDB project, and adds the reference to the OpenFlow plugin
project.

Change-Id: I27845139039e70eae4f976eddbc495a83759aa0d
Signed-off-by: Thomas Bachman <tbachman@yahoo.com>