support mediawiki/core under $WORKSPACE/src/mediawiki/core

MediaWiki core has been fetched under $WORKSPACE directly which is a bit
of a mess.  This patch add supports to have it fetched using
$ZUUL_PROJECT and under $WORKSPACE/src/

Introduce a new script mw-set-env.sh which sets MW_INSTALL_PATH to
either WORKSPACE or WORKSPACE/src/mediawiki/core. Update various slave
scripts to use MW_INSTALL_PATH.

Change-Id: I4e72c92118c56506fee3316d3d78aa6dd50fde31