ovn: l3ha, make is_chassis_active aware of gateway_chassis

ovn: l3ha, make is_chassis_active aware of gateway_chassis

is_chassis_active now is only true for redirect-chassis ports
in the case of the specific lport being active on the
local chassis.

This will naturally make the ARP responder / redirection openflow
rules automatically inserted/removed when a router goes active/backup
in a way that BACKUP routers won't respond to ARP on gateway ports,
and they won't route packets that arrive on the wrong gateway
chassis (that can happen until all hypervisors converge in the
new MASTER thanks to the BFD monitoring of the tunnel endpoints).

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