aptrepo: mv wikimedia-specific distributions file to role
This file is specific to wikimedia and should not be realized
in the module. The file can be there but the role class should invoke
it. It's needed for aptrepo::wikimedia.
When, like on bromine, aptrepo::distribution is used, this creates a
conflict and puppet error.
It could also be replaced with aptrepo::distribution instead but
some options are missing for now.