contint: publish Zuul git repositories

When running jobs on slaves, we need the Zuul reference it crafts.  This
can be done by publishing them with git-http-backend bound to the URL
http://integration.wikimedia.org/zuul/git/

This is limited to the internal network for now, localhost and gallium
public IP address (the server has a Jenkins slave). Also it is done over
HTTP instead of HTTPS to save up a bit of SSL processing.

bug: 50695
Change-Id: I0248003dc3564d4d2fce63f962517a0220b36300
Merge "contint: publish Zuul git repositories" into production