Switch to using [DEFAULT]transport_url for configuration of
oslo.messaging connectivity to RabbitMQ.
Use section-oslo-messaging-rabbit to set [oslo_messaging_rabbit]
sectional configuration options for HA and SSL.
Update amulet test to only run ceilometer upgrade in deployments
with Gnocchi (>= Xenial/Queens).
Change-Id: If0ee7a09bac3bb4c5e6823adb1994ad3a120f9d5
Closes-Bug: 1817672