ovn: l3ha ensure no master bouncing when ovn-controller is restarted

ovn: l3ha ensure no master bouncing when ovn-controller is restarted

When ovn-controller is restarted, ovn-controller removes the old
Chassis entry from the SBDB and a new one is inserted.

This cleared the Gateway_Chassis chassis column in the SBDB and then
ovn-northd removed the empty-column Gateway_Chassis entry.
Such event made the other (non-restarted and master gateway chassis)
believe that he was a single (non-HA) gateway, turning off BFD and
releasing the port for a tiny time frame causing unnecesary downtime.

Signed-off-by: Miguel Angel Ajo <majopela@redhat.com>
Signed-off-by: Russell Bryant <russell@ovn.org>