Start and stop containers as required to match configuration

[adamtheturtle]

#251 will output objects describing necessary changes which must take place to match a given configuration. This issue covers the remaining subset of #33 which involves performing those actions.

[adamtheturtle]

We had a start at some of this in #251 and scrapped it, but some ideas which may help:

In the ChangeStateScript class, have _deployer = Deployer() and test that this is the case with something like:

    def test_deployer_is_deployer(self):
        """
        ``ChangeStateScript._deployer`` is an instance of :class:`Deployer`.
        """
        script = ChangeStateScript()
        self.assertIsInstance(script._deployer, Deployer)

Also, test that some part of Deployer.change_node_configuration is used by ChangeStateScript.main by testing for example that when a fake _deployer is set with Gear units running, and an empty configuration is sent to the change-state script, the right changes happen.

[adamtheturtle]

Also, we found that the desired configuration should be set in parseArgs for the change-state script with Configuration.model_from_configuration

[adamtheturtle]

ChangeState.main should also require a hostname (which will be the hostname of the node it is run on, covered by #19), and pass that as a parameter to Deployer.change_node_configuration

[adamtheturtle]

Some of this doesn't need #251 so I will start it now.