Simplify the way we pass dependencies to ext jobs

Since Jenkins Job Builder 0.8.1, we can pass additional variables.  That
let us dramatically simplify the way we define dependencies for
MediaWiki extension jobs.

For reference upstream change is:
  (https://review.openstack.org/#/c/85383/

Change-Id: I63ea2eeb2f22faa1e63dd50d0d5322517a514d1a