contint: publish Zuul git over git protocol

This is similar as ed2fb38 https://gerrit.wikimedia.org/r/#/c/71968/
which published the Zuul repositories over HTTP using git-http-backend
for bug 50695.  It turns out the process hangs (bug 53683), so here we
publish the Zuul repositories over the git protocol.

* git-daemon-sysvinit package provides all the glue around git daemon
  command.
* GIT_DAEMON_DIRECTORY and GIT_DAEMON_BASE_PATH are both pointing to
  Zuul git root path. Let us have URL without a /git/ prefix in the
  path.
* git-daemon port is firewalled to prevent access from outside.
* added git_daemon to iptables port and protocols tables.

bug: 50695
bug: 53683
Change-Id: I0c50e5dc2899c3f23a875309e1920ee4d51551cf
Merge "contint: publish Zuul git over git protocol" into production