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.
There's already a helper to get a sample environment config 
See provider/dummy package

example usage:

	cfg, err := config.New(config.NoDefaults, dummy.SampleConfig().Merge(
		testing.Attrs{
			"state-server":   false,
			"name":           "erewhemos",
			"ca-cert":        string(testing.CACert),
			"ca-private-key": string(testing.CAKey),
		},
	))
	c.Assert(err, gc.IsNil)