aptrepo: mv wikimedia-specific distributions file to role
This file is specific to wikimedia and should be realized in the role
class not the module itself.
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.
Bug:T132757
Bug:T136793
Change-Id: Ib98634da6745278ab0948ddb14aaf69434fc93a4