nicira: Add RoleReply event

This adds an event for role replies.  This patch is based on the one by
idryzhov in issue #103, but with a minor improvement, a minor bugfix,
and updated to apply against current dart.

Thanks to idryzhov and Denis Garyachy for their assistance.
Remove openflow.nicira_ext

openflow.nicira has completely subsumed nicira_ext.