Add support for connecting to an existing message bus

Currently the plugin will configure and deploy a message bus using the released packages for either qpidd and/or qdrouterd.

This feature would skip the whole message bus deployment and configuration step and instead connect to a pre-provisioned message bus.

This feature is intended for developers who want to run devstack against experimental or pre-released versions of the messaging backend. The developer would set up the desired messaging bus and simply tell the plugin the addresses used to access the bus.