ovn: Add '--restart' flag to ovn-controller exit.

ovn: Add '--restart' flag to ovn-controller exit.

When "--restart" is passed to ovn-controller's exit command, then
database entries are not removed for this hypervisor. This means that
* Encaps
* Chassis
* OVS ports
are not removed.

The reasoning is that if the intent is to restart ovn-controller, this
will allow for tunnels to remain up and allow for traffic not to be
interrupted during the restart. When ovn-controller is started again, it
picks back up from where it was.

Signed-off-by: Mark Michelson <mmichels@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>