Fix 500 raised on disassociate_floatingips when out of sync

If one deletes NAT rules directly from NVP and not through quantum,
an error will be raised when deleting a port that is associated with
the floating ip. This patch catches the NotFound exception and logs
it so that the port can successfully be deleted.

Fixes bug: 1177176

Change-Id: Ia7d0b94957d0c6b014d242439245b0e4fa9de40d