KVM containers can now be created using a user-provided SimpleStreams URL as a base.

First step to alleviating https://bugs.launchpad.net/juju-core/+bug/1322302. Second PR to follow which will make apt-get behavior more configurable.
Done.

I also do not like this pattern of introducing package-global variables just for tests. I attempted to write a more black-boxed approach, but it turns out we are mocking the containers, and so without creating more mocks (which I take to be the antithesis of black-box testing), I can't do this. Given that I'm sticking with the more unit-test style. With this approach in mind, I felt it important to:

1) Test that CreateContainer(...) is calling container.Start(...) correctly.
2) Test that container.Start(...) is calling kvm.SyncImages(...) correctly.

The tests in libert_test.go then take over.