gitblit: fix monitoring check

gitblit (git.wm.org) is behind the misc-lb Varnish cluster nowadays
which handles SSL termination for the service as well. The machine
itself doesn't respond to HTTPS (in fact, it just happens for some other
service, svn.wm.org, to be responding on 443).

Fix the check to check HTTP instead of HTTPS and hence making the check
work again, rather than hitting svn and 404ing.

Change-Id: I0a9689973a89284e71add8c6bd421bd9f600c28d
Merge "gitblit: fix monitoring check" into production