Fix rabbitmq reconfigure, simplify role

Since Id724b44a3edd951fa8b06c9f2c347e9ed8c5ffd9, there is a reference to a
non-existent variable, rabbitmq_confs, that causes deployment to fail if
rabbitmq configuration other than config.json is changed.

I'm taking this opportunity to simplify the role, since we can use the Ansible
handler notification system to determine when handlers need to run, without
registering and checking variables. This simpler approach was used in the
haproxy refactor.

Change-Id: Ibe0e7fda93afff741243ff9c350db1c8c6e1e6d3
Closes-Bug: #1816053