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.
This isn't right - ImageStream() returns "released" or "daily" etc. It needs to be turned into a URL like "http://cloud-images.ubuntu.com/daily"
See man uvt-simplestreams-libvirt
And in the case where the image stream is "released" (the default), the URL needs to be .../releases