Zuul cloner for MediaWiki extensions

Use Zuul cloner for extensions!

The new jobs are named 'mwext-Foobar-testextension'.

The list of dependencies are the extensions base name and are passed to
the job template via jjb variable 'dependencies'.  We use some inline
bash to expand them to the full Gerrit project name.

Requirements:

- make sure extensions release branches pass with the appropriate
  MediaWiki core version (REL1_22 and REL1_23) instead of master which
  have been using until now.

Change-Id: I07db61d567e54827ee7830880ac5d2e78f9fadaa