l2-population : send flooding entries when the last port goes down

Delete port used to call update_port_down to calculate
its fdb entries during delete_port_precommit. But during
the pre-commit, the port is still up, so update_port_down
acts as if there was still one port on the agent, and
doesn't add flooding entry in its fdb_entries.

Closes bug #1282141

Change-Id: Icedd02f33d999200505950beb9169ae3634e551b
(cherry picked from commit 043b4dde00216e71e855ff353e41695e4ed940a9)
Merge "l2-population : send flooding entries when the last port goes down" into stable/havana