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.
Out of curiousity; is there a reason we can't do this block of code in the for loop in a goroutine so that the machines can all be provisioned asynchronously rather than serially?