Update Wikibase, use siteGroup setting instead of doing lookup

backports I9d52070 into deployment branch of Wikibase

re-introduces siteGroup Wikibase setting which can be explicitly set,
for improved performance and reduce memcached load.

Bug: 56602
Change-Id: I88dbf4b3056c06c3b9bc7975ecc3ac1eb86c679c