Big Switch: Send notification after port update

The Big Switch plugin was not sending port update
notifications for security group changes after the
port was updated. This patch corrects that behavior
by calling the port_update method on the RPC notifier
when necessary after a port update.

Closes-Bug: #1376527
Change-Id: Ic2fd872725c1da8e2dc394473b7feb407d21268f
Merge "Big Switch: Send notification after port update"