Configurable apt-get behavior.

For:
https://bugs.launchpad.net/juju-core/+bug/1322302
https://bugs.launchpad.net/juju-core/+bug/1350493
Actually, I've removed these. If we include defaults, I don't think the Validate method can determine if the value is present because of the default, or because the user specified it in the environments.yaml. If omitted, the code in config.go takes over, and Validate works properly.