Switch extensions qunit jobs to Zuul cloner

Instead of testing with MediaWiki@master use Zuul cloner \O/

Mark the default 'use-zuul-for-mw-ext' as obsolete. Still being used by
the Wikibase repo though.

The VisualEditor ends up being broken because it fetch the VisualEditor
project has a submodule under /lib/ve. We need a dedicated job that
would be able to be triggered on both repositories.  That will be
addressed in a follow up patchset.

Updates:

mwext-ContentTranslation-qunit
mwext-DataValues-qunit
mwext-Echo-qunit
mwext-EventLogging-qunit
mwext-Flow-qunit
mwext-GettingStarted-qunit
mwext-GuidedTour-qunit
mwext-ImageMetrics-qunit
mwext-Mantle-qunit
mwext-Mantle-qunit-mobile
mwext-MobileFrontend-qunit
mwext-MobileFrontend-qunit-mobile
mwext-MultimediaViewer-qunit
mwext-Popups-qunit
mwext-TemplateData-qunit
mwext-UploadWizard-qunit
mwext-VisualEditor-qunit

Change-Id: Ie23ca07697a61543fd9b7c1e8c5a195f849cee25