[RFE] add juju config for selecting series of container

[Zachary Zehring]

It would be nice to be able to configure some of the properties of the juju environment/controller. For example, charm-juju-controller [0] uses juju-local for creating an inner controller/model and running charm tests against the inner environment. One of the tests depends on the series of the controller (specifically the controller machines). So to properly test this, charm-juju-local would have to provide a way to set the series to test both flows.

[0] https://launchpad.net/charm-juju-controller