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.
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// Exposed for testing
or similar so the reader knows why this have been pull out of the method.
It's a shame to have to do this, I'd prefer a more black box approach where the external behaviours are tested eg the resulting uvt command