HA routers master state now distributed amongst agents

We're currently running with no pre-emption, meaning that
the first router in a cluster to go up will be the master,
regardless of priority. Since the order in which we sent
notifications was constant, the same agent hosted the
master instances of all HA routers, defeating the idea
of load sharing.

Closes-Bug: #1365429
Change-Id: Ia6fe2bd0317c241bf7eb55915df7650dfdc68210
Merge "HA routers master state now distributed amongst agents"