citrain tool should return apt to its previous state after installing the silo

To install the silo, the citrain tool runs 'apt-get -o Dir::Etc::SourceList=/dev/null update'. This makes it so that apt will only install packages from the PPA specified. This is obviously a good idea, but it might be good to revert this change after installing the silo, that way the tester is able to install any further packages they need without needing to run apt-get update again (which they may not realise they need to do). If we do this for them after installing the silo packages then I think it will be more friendly to the tester.