Update rabbitmq password if it's different

This works by checking we can connect with current credentials, using
the API offered by the management plugin.
Enable management plugin on SUSE

We simply need to install the rabbitmq-server-plugins to get the plugin.
Put mochiweb port in non-configurable attribute

This is better than having it hard-coded in several places.
Correctly set default port attribute in cookbook
Correctly log only when an action is done
Rework how we automatically set a password

Do it in the same way as for database passwords.
Merge pull request #12 from vuntz/fix-new-password

Fix re-creating rabbitmq proposal