Configurable apt-get behavior.

For:
https://bugs.launchpad.net/juju-core/+bug/1322302
https://bugs.launchpad.net/juju-core/+bug/1350493
That comment was in regards to the global config variable, prior to calling Manual's Validate method. These tests are more appropriate in provider_test.go, so I have deleted these in favor of equivalents there.