Zuul initial clonemap for MediaWiki

The Zuul cloner utility https://review.openstack.org/#/c/70373/ is
deployed on gallium.  We can define destinations of a repositories in a
YAML file, this one should be good enough for MediaWiki extensions,
skins and vendor repositories.

Change-Id: I10928c4a1d5c76ec29d4d860e9afd5212fba89d4