Salt uses the aptsources system library to get a list of repositories on
the host. Unfortunately if the /etc/apt/sources.list file is missing,
aptsources.sourceslist.SourcesList() (which is called by salt) logs a
warning, which is then displayed to the user running salt.

To avoid this warning when using 'remove_sources_list: true' in pillar
we can keep the /etc/apt/sources.list file, but just ensure it is empty.
This achieves the same thing, but does not log the warning or display it
to the user.