provider: Restore the Ubuntu default sources.list
provider: Restore the Ubuntu default sources.list

We disable it for the duration for which we're using `apt-get update`
and retore it afterwards so users have access to it later.

mmussomele/quilt,tejasmanohar/quilt,secant/quilt,ejj/quilt-dev,tejasmanohar/quilt,PenguinToast/quilt,stevenlin598/quilt,stevenlin598/quilt,LuiseV/quilt,akshayknarayan/quilt,NetSys/quilt,kayousterhout/quilt,kayousterhout/quilt,kklin/quilt,LuiseV/quilt,PenguinToast/quilt,quilt/quilt,mmussomele/quilt,kklin/kelda,quilt/quilt,ZhongtianWang/quilt,secant/quilt,stevenlin598/quilt,LuiseV/quilt,tejasmanohar/quilt,tejasmanohar/quilt,PenguinToast/quilt,mmussomele/quilt,stevenlin598/quilt,stevenlin598/quilt,kklin/kelda,ZhongtianWang/quilt,LuiseV/quilt,kelda/kelda,akshayknarayan/quilt,ZhongtianWang/quilt,ejj/quilt-dev,kklin/kelda,NetSys/quilt,kelda/kelda,LuiseV/quilt,kelda/kelda,NetSys/quilt,mmussomele/quilt,PenguinToast/quilt,mmussomele/quilt,ejj/quilt-dev,kklin/quilt,quilt/quilt,mmussomele/quilt,quilt/quilt,kklin/kelda,secant/quilt,secant/quilt,kayousterhout/quilt,ejj/quilt-dev,kayousterhout/quilt,akshayknarayan/quilt,kelda/kelda,kklin/quilt,kklin/quilt,NetSys/quilt,secant/quilt,PenguinToast/quilt,stevenlin598/quilt,ZhongtianWang/quilt,NetSys/quilt,akshayknarayan/quilt