provision/juju: fix intermitent test

By making the query more restrictive, we made this test more reliable.
provision/juju: small variable extraction

Just avoiding very long lines.
provision: removed RemoveUnits method from the provisioner

The package app is resposible for iterating and removing units.
testing: removed RemoveUnits from FakeProvisioner

This method does not exist anymore in provision.Provisioner.
provision/juju: removed RemoveUnits method

provision.Provisioner interface does not require this method anymore.