Experimental config switcher for WMF Jenkins jobs

Wikibase would need a client and a server job that would let tweak the
default configuration settings.  I have created two Jenkins jobs that
launch the full PHPUnit file and named:

- mwext-Wikibase-client-tests
- mwext-Wikibase-server-tests

Jenkins provide the job name as the JOB_NAME environnement variable. One
can thus detect which job is being run and tweak the PHPUnit setting.
For the client job, we insert a `--exclude-group WikidataClient`.

This also includes an evil hack to populate the sites table since
there are some tests that require this.

Change-Id: Ide41a0b9003a9627f5a86c0514cc61d684b5e6aa