Configurable apt-get behavior.

For:
https://bugs.launchpad.net/juju-core/+bug/1322302
https://bugs.launchpad.net/juju-core/+bug/1350493
The logic we want to use here is:
if DisablePackageCommands is true -> set both update/upgrade to false
if DisablePackageCommands is false -> set update/upgrade to whatever the config settings say they should be