[1chb1n, r=thedac,gnuoy] Add amulet & openstack/amulet helpers for rabbitmq-server tests; resolve misc. race conditions in amulet helpers.
Add exception handling to the retry logic in service_restarted_since re: bug 1474030.
Add deprecation WARN to the service_restarted amulet helper.
Update _determine_branch_locations, add a new force_series_current mechanism. ie. Always use trusty/nrpe instead of precise/nrpe, even when on series precise.
Note re: validate_sectionless_conf - plucked and generalized from gnuoy's openstack-dashboard amulet test. rabbitmq confs have a similar need. Will update the openstack-dashboard test to use this helper once this lands.