provision/juju: generalize cleanQueue function used in tests

I will move it to testing package later, and use in all packages that
needs to clean the queue.
provision/juju: don't enqueue instances that were pending

Just avoiding double queueing of this message.
testing: added CleanQueues function

As the name suggests, this function cleans multiple queues, identified
by their names.
provision/juju: use testing.CleanQueues function
app: use testing.CleanQueues to clean queues