Deletes FIP agent gw port when last VM is deleted

This patch deletes the FIP agent gateway port in
the plugin, when the last VM with FIP using this
FIP agent gw port on a Compute node gets deleted.

Closes-bug: #1367588

Change-Id: I71a9fa48544fa4bbd08ac6d83c99e6542c818009
Merge "Deletes FIP agent gw port when last VM is deleted"