Change ordering of ceilometer service startup

If the compute-agent starts before the collector directly
after a start or restart of the AMQP service, samples published from
the compute-agent can be lost before the collector has had a chance
to establish connections. These lost samples impact the reliability
of tests which run immediately after the service [re]start.

Note: if there is a restart of the ceilo service, but not the AMQP
service, the problem does not present itself becaue the messaging
service maintains some state on the exchanges it keeps.

Change-Id: I1c06d0511fbf93050cda56d9d2de0ff00813dfb6
Closes-bug: 1355809