Allow enabling authentication on haproxy.stats

Allow enabling authentication on haproxy.stats

Right now we always deploy the haproxy.stats endpoint with no
authentication, which is a security concern.  Allow setting a
password on the endpoint so it isn't accessible to the world.

While this allows configuring SSL on the stats endpoint, it does
not use the service_certificate parameter because that certificate
is intended to be used only for public endpoints, and the stats
endpoint is actually on the admin VIP.  Once we have support for
SSL on admin endpoints we can have stats use it by default.

Change-Id: I8a5844e89bd81a99d5101ab6bce7a8d79e069565