ha: Make neutron-ha-tool start after haproxy and neutron-server

Otherwise, neutron-ha-tool fails to start.

Of course, we only do that when neutron-server and neutron-l3 roles are
on the same cluster, and both with HA.
Merge pull request #96 from vuntz/order-constraints

ha: Make neutron-ha-tool start after haproxy and neutron-server