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.
Is this OK? Should the code be calling environ.FinalizeMachineConfig(...) in addition to this? The comments said only to call it if a machine was bootstrapped, and I don't think we're bootstrapped at this point.