Implement ActiveMQ authentication mechanism

We need an authentication mechanism for ActiveMQ so that we can manage external connections from Remote Pollers, Minions, and other external processes.

ActiveMQ doesn't include an authenticator that works directly with Spring Security but it should be easy to write one. Ideally, we could connect to a Spring Security AuthenticationManager bean in the context that is shared with the OpenNMS Web UI. Users returned from this authenticator could support a role that is specific to ActiveMQ access, ie. ROLE_MESSAGING.