Code review comment for lp:~hopem/charms/precise/rabbitmq-server/lp1281753

I have a revised version of this branch which switches the install steps from install to config_changed, which means that a change in source will take effect during operation of the service

For example, the rabbitmq in 12.04 can be replaced with the rabbitmq-server from the cloud-archive for icehouse.

Note that the upgrade can be racey - as it happens across all service units at the same time so if rabbit is clustered this can cause hook failures which can be resolved --retry'ed