A HA router should always be active unless all of the agents hosting
that router go down. In that event, the router should switch to
standby. This behavior changed with review:
https://review.openstack.org/#/c/411784
That review seemed to be accounting for a flakey message bus. This
change should account for that, but also revert to the original behavior
of the router state only changing when its backing agent hosts are down.
Change-Id: I89c3b2546382624f175f8de4de621c3e53adf527
Closes-Bug: 1682145
b62d1bfdf7