Add CentralAuth role

This role installs the CentralAuth and AntiSpoof extensions and creates
two additional wikis: login.wiki.local.wmftest.net is the login wiki and
centralauthtest.wiki.local.wmftest.net is configured to show that logins
work automatically across the wiki farm.

Co-author: Bryan Davis <bd808@wikimedia.org>
Bug: 52306
Change-Id: Ide7acba8cd6a223d38cb26b766a6d9482905c5fa