Delete FIP namespace when last VM is deleted

On a compute node when the last VM with a floating IP association
is deleted, the L3 agent did not delete the fip namespace. However
the api server has already deleted the fip agent external gateway
port from the database.

This problem is happening on DVRs because the deletion of a VM port,
in addition to a floating IP disassociation, may also result in the
removal of the external gateway port binding AND the removal of the
fip agent external gateway port.

When the L3 agent is handling a routers_updated notification, it is
not processing floating ip address updates when the router has both
a floating ip disassociated and a external gateway port deleted.
This patch corrects this problem.

cherry-picked from e3b949c3bc08808e3df15215bc30d6610f3a4bd3
Closes-bug: #1377156
Change-Id: I86bdef7c9d988cb9d87c88adde55548d459f29a5
Merge "Delete FIP namespace when last VM is deleted" into stable/juno