Don't depend on l3-agent running for IPv6 failover

Right now, we're enabling IPv6 RA on the gateway interface for master instances [0].

This happens only when the l3-agent is running so we depend on it for the correct configuration of the HA routers.

If the l3-agent is shut down for maintenance, then we won't get RA enabled on master instance even though keepalived and keepalived-state-change are both running.

We should get rid of this dependency by moving this code into keepalived-state-change, which we could assume that will be always running along with keepalived.

[0] https://github.com/openstack/neutron/blob/master/neutron/agent/l3/ha.py#L124