Parametrize MediaWiki install path
Parametrize MediaWiki install path

I don't expect the value to be commonly overridden, but having a reference to
the value allows for writing more expressive Puppet code that interacts with
the MediaWiki setup, as the next commit will show.

Change-Id: I0892e18d585660079998de3bc8e0570b82d4fbae

stuartbman/mediawiki-vagrant,stuartbman/mediawiki-vagrant,huit/mediawiki-vagrant,ankita-kumari/mediawiki-vagrant,ankita-kumari/mediawiki-vagrant,huit/mediawiki-vagrant,physikerwelt/DRMF-vagrant,wikimedia/mediawiki-vagrant,stuartbman/mediawiki-vagrant,physikerwelt/DRMF-vagrant,wikimedia/mediawiki-vagrant,wikimedia/mediawiki-vagrant,stuartbman/mediawiki-vagrant,HarriAlasi/wiki3dviewer,physikerwelt/DRMF-vagrant,HarriAlasi/wiki3dviewer,wikimedia/mediawiki-vagrant,HarriAlasi/wiki3dviewer,HarriAlasi/wiki3dviewer,ankita-kumari/mediawiki-vagrant,ankita-kumari/mediawiki-vagrant,physikerwelt/DRMF-vagrant,huit/mediawiki-vagrant,huit/mediawiki-vagrant,stuartbman/mediawiki-vagrant,HarriAlasi/wiki3dviewer,huit/mediawiki-vagrant,stuartbman/mediawiki-vagrant,ankita-kumari/mediawiki-vagrant,ankita-kumari/mediawiki-vagrant,HarriAlasi/wiki3dviewer,wikimedia/mediawiki-vagrant,stuartbman/mediawiki-vagrant,wikimedia/mediawiki-vagrant,HarriAlasi/wiki3dviewer