Ensure rabbitmq users exist when processing access requests

[Alvaro Uria]

Initial status:
* 3 rmq units clustered and related to OpenStack services
* rmq users created when the application was related to other OpenStack applications (nova-cloud-controller, neutron-api, etc.)

Step to reproduce failure:
* juju remove-unit <all 3 units> # ie. because mnesia db is corrupted and we are unable to restart the cluster
* juju add-unit --to lxd:host1
* juju add-unit --to lxd:host2
* juju add-unit --to lxd:host3

Problem:
* New rmq cluster does not have rmq users (amqp-relation-changed hook does not recreate them)

Workaround:
* juju remove-relation rabbitmq-server:amqp <openstackservice>:amqp
* juju add-relation rabbitmq-server:amqp <openstackservice>:amqp

Request:
* Juju action to trigger rmq user recreation (ie. by relation or "--all"), to avoid the remove/add relation dance

Thank you.

[James Page (james-page)]

I really think the charm should just do the right thing; when the new units appear and one is elected leader, it should figure out that the users don't exist and re-create them (I suspect the check is done on presence of data in the leader database provided by juju).