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.
There isn't an existing tests for the start params generation as far as i can see so if we add one here, best to check for all attributes, not just image download url.