As found in Ie5d55b2a2d96a78b34d23cc6fbac62900a23fc37, the default for
this is to issue "OPTIONS /" which is kind of a weird request. The
Zuul hosts currently seem to return the main page content in response
to a OPTIONS request, which probably isn't right.
Make this more robust by just using "HEAD /" request.
Change-Id: Ibbd32ae744af9c33aedd087a8146195844814b3f
29202eba1a