adt-run: Reset changes made to the apt configuration, on exit

adt-run: Reset changes made to the apt configuration, on exit

After this change running
  adt-run --quiet --no-built-binaries hello_2.6-1.dsc --- adt-virt-null
  find /etc -name "*autopkgtest*"
does not print any files.

[ Ideally we would leave these config file settings somewhere where
  apt won't find them unless it's told to.  That way if adt-run crashes
  for any reason, we wouldn't be relying on this cleanup code, and we
  could remove it.

  However, when I tried to do this I found I couldn't get apt to
  reliably pick up all the files I needed using command line options.
  So this is an improvement for now.  -iwj ]