Replace Celery with Oslo messaging for asynchronous queuing.

Barbican is using Celery to interface to the HA queues it uses to send
asynchronous requests from API nodes to the worker nodes. This change
request replaces Celery with OpenStack-friendly oslo.messaging RPC
components.

Change-Id: Ibc257e98726938a577da4300b389fa13ff34d8a0
Implements: blueprint queue-use-oslo-messaging