Configurable apt-get behavior.

For:
https://bugs.launchpad.net/juju-core/+bug/1322302
https://bugs.launchpad.net/juju-core/+bug/1350493
Corrected, thank you! Also added code in validator to ensure false, and two tests: 1) check that if not specified, defaults to false, and 2) defaults will be ignored if overridden in environments.yaml.