Add support for testing pebble services interaction

[tonyandrewmeyer]

There is currently no way in ops-scenario to test a charm interaction with a pebble service, like restarting it for example: self._container.restart(self._service_name).

The only way to accomplish this is to mock ops.model.Container.restart.

It would be nice to have a better way to do this, maybe as an automatic mock in the output state?

...

state_out = self.ctx.run(container.pebble_ready_event, state_in)

assert state_out.containers[0].services["ausf"].restarted

Moved from canonical/ops-scenario#34