trove related enhancements

This adds some enhancements that are needed by the trove barclamp. We allow (default off) to enable access to rabbitmq via the public network so that the trove-guest-agent can reach it.

Note: This has some security implications. As it doesn't seem possible to restrict access to rabbitmq vhosts by IP address (only by user). All vhost/queues will be accessible on the public network, provided you know a valid username/password combination for that vhost :(

This is still missing a few things, which will follow soon:
* creating a special vhost, a user and setting permissions for trove
* testing if this works with HA enabled
* UI changes (I guess these can be optional as trove support is just a Tech preview)
 

@vuntz I know, but the trove attribute is defined within the rabbitmq template: https://github.com/crowbar/barclamp-rabbitmq/pull/53/files#diff-159c509110e5b144e20d0b1a4cc9633dR24

And i've got to say it feels some kind of wrong to put this attribute into rabbitmq.