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.
Testing log messages is yucky, even though we do it elsewhere. There's test infrastructure in utils/command or somewhere like that which allows the command to be stubbed out and the command line captured and that's what we should be testing against