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.
Just to recap the discussion in IRC: this is a _great_ idea, and something that could really help speed up the provisioning process. Unfortunately there is a bit of ambiguity about a potential bug, and this kind of optimization would be better suited for another PR. Still, thanks so much for the great idea.