Change HorizonSecureCookies default to False

HorizonSecureCookies is incompatible with non-ssl deployments, which
is our default deployment method.  When SSL is in use, it can be
turned on in the enable-tls.yaml file.  This does mean that
existing users won't automatically get this feature turned on as
part of their upgrade because enable-tls.yaml is an environment that
is intended to be copied and edited, but it's simple to add the
parameter to the file for users who want that behavior after they
upgrade to a version where it is available.

Change-Id: If83d3d8709fc4e0c09569e8bf524721d332bf560
Closes-Bug: 1696861