Configurable apt-get behavior.

For:
https://bugs.launchpad.net/juju-core/+bug/1322302
https://bugs.launchpad.net/juju-core/+bug/1350493
Corrected, and I added an extra permutation to TestProvisioningScriptDisablePackageCommands to check for the case where DisablePackageCommands is false, but the settings from environments.yaml are also false.