For some reason test cases are sometimes executed out of order and so
test_921_remove_unit is sometimes run before the pause_and_resume test
case which results in an error.
While the root cause for it must be found it would also be good to avoid
side-effects in individual test cases and return the environment back to
its original state.
There is no 'start' hook implementation for charm-rabbitmq-server,
however, changes that close to the 20.05 release are discouraged so this
change uses an upgrade-charm event simulation to re-trigger the addition
of a unit (which was previously removed) to the cluster.
NOTE: after an execution of hooks/upgrade-charm finishes, the charm will
stay in the waiting state with the following status until the next
update-status event: 'Unit has peers, but RabbitMQ not clustered'
Related-Bug: #1730709
Dmitrii Shcherbakov
dfc6235362