Configurable apt-get behavior.

For:
https://bugs.launchpad.net/juju-core/+bug/1322302
https://bugs.launchpad.net/juju-core/+bug/1350493
Hmmm. I'm very tempted to recommend using schema.Omit and have each provider fill in the values it wants to use for defaults explicitly. There's too much danger we could allow subtle bugs to be introduced by a developer doing a config.New(config.UseDefaults, attrs) and then using the values directly.