Support qpid as message provider for ceilometer

Previously, the RPC backend was hardcoded to the appropriate
dirver for rabbitMQ only.

Now we allow qpid to be used as an alternative.

Note that ZeroMQ is not tested or supported as yet.

Change-Id: If9a69a4c52f046fc6e3530b47542d55438787cea
Merge "Support qpid as message provider for ceilometer"