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.
Here be dragons.

- When we're provisioning, in some situations we want to know about all the other related instances (for automatic availability zone placement).
- Rather than provisioning N instances asynchronously, it would be preferable to synchronously make a single bulk provisioning call for N instances to the provider API. This will cut down on IaaS API usage, avoiding rate limiting.