Experiment extensions with Zuul cloner

A new kind of jobs mwext-Foobar-testextension are using the Zuul cloner
to clone:
- MediaWiki core
- MediaWiki vendor
- extension
- extension dependencies as defined in JJB

For each repository, it attempts to checkout the branch of the
triggering patch then run the tests.

Culprits:
- tests are most probably broken on wmf and REL branches
- REL branches ends up using vendor@master though they should not depend
  on it.
- all repos are a full clone which eat a ton of disk space

Change-Id: I1a4fb20f96f895ea771ac3901a341d02c9aca410