[ovn-ha] ctl: neutron: Add msg cluster pillar

[ovn-ha] ctl: neutron: Add msg cluster pillar

`system.neutron.control.cluster`, unlike
`system.neutron.control.openvswitch.cluster` does not provide
neutron:server:message_queue:members pillar data, letting it
default to the wrong values (single rabbit instance on the same node).

This led to neutron.conf on ctl nodes using:
`transport_url=rabbit://openstack:***@172.16.10.35:5672//openstack`
instead of
`transport_url=rabbit://openstack:***@172.16.10.28:5672,...//openstack`

Change-Id: Iad4b709d555b2bafafeb75fdecb831f7d4f5a504
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>