Corrected default Parameter / Migration to puppetlabs-apt usage

I hope you agree to merge the two changes:

1.  The ``$master_nodgroups`` parameter should have a default of ``undef``, otherwise the master template creates a unnecessary nodegroup entry, when the parameter is an emtpy hash.

2. The common module to use for handling apt repositories is ``puppetlabs-apt``. So I converted the ``salt::repo::deb`` class to the corresponding resources.