settings injection for MediaWiki

That is like the 3rd time I hack a way to inject settings in a MediaWiki
installation.  We used /jobs/_shared/ExtraSettings.php which was
injected using a ant script, eventually started inlining shell script
directly inside Jenkins jobs.  Now we get some kind of a mediawiki
conf.d system :-]

Folks can add PHP snippets in mediawiki/conf.d. It will be made
available on CI Jenkins slave under /srv/slave-scripts/mediawiki/conf.d.

The filenames must start with two digits and their content will be
injected in alphabetical order at the bottom of LocalSettings.php.  The
way it is done, files MUST not ends with a PHP closing tag, it is
appended by the _join.php script.

The settings injection in bin/mw-install-sqlite.sh has been removed, one
should instead use the new bin/mw-apply-settings.sh script to inject
extra settings.

Change-Id: If3b37cef3b149323516c44b27836f5a4e59bd348
extension: set MW_INSTALL_PATH to WORKSPACE when unset

The Wikidata extension has its extensions under
/extensions/Wikidata/vendor/* .. That causes a bunch of issues when
using relative paths.

JJB equivalent has same Change-Id and is:
    https://gerrit.wikimedia.org/r/#/c/89168/

Change-Id: I5ad86e694963098f2a98cbca0abec9eed29bc9c1