beta: bring in mediawiki/skins.git

Some skins have been migrated out of MediaWiki core to standalone
repositories under mediawiki/skins.  Gerrit aggregate all of them in the
mediawiki/skins.git much like mediawiki/extensions.git

I slightly refactor wmf-beta-autoupdate.py to have extensions and skins
updates to share the same code.

The puppet change is a bit nastier. MediaWiki core has a /skins/
repository in which we want to clone mediawiki/skins.git. git clone will
refuse to proceed since the directory exist.  Instead do it in a two
steps process: 1) git init  2) git remote add.  Then we use git::clone
which takes care of pulling the repository.

Bug: 65868
Change-Id: I494d86208975ccb96ee62d00cf7a1c57da0f89ca