Change the queue 'broker' config to allow a list of brokers.

For high availability, multiple concurrent queues are required. Hence
a list of brokers could be provided in the Barbican configuration file.
This commit accommodates treating the 'broker' config attributes as a
comma-delimited list of brokers.

Change-Id: Ia48e6483835acc518014f0270bb180e5a5e9f1dd