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.
It looks like provisionerTask is intended to be used with goroutines. Further, it looks like AgentConfig is possibly meant to contain config settings that need to be thread-safe for a machine. Is it OK for a provisionerTask to contain an environConfig?