Configurable apt-get behavior.

For:
https://bugs.launchpad.net/juju-core/+bug/1322302
https://bugs.launchpad.net/juju-core/+bug/1350493
The manual provider's Validate method observes that the values are only true because they are defaulted, and thus sets the correct default to false. Per Andrew's email, I'll have to change one of the defaults to true.