Fix openstack-dashboard.conf in horizon SSL conf

As reported by danieru, who also provided this fix, the
openstack-dashboard.conf in the section on
"Configure the dashboard for HTTPS" was incorrect, and did
not work for versions of apache included in current
distributions.

Change-Id: I12d38945268589c0d988220460d8ab0a02c41354
Closes-Bug: 1321660
Merge "Fix openstack-dashboard.conf in horizon SSL conf"