Enable support for secure cookies

Secure cookies are recommended for public usage over https. They are
documented here:

http://docs.openstack.org/developer/horizon/topics/deployment.html#secure-site-recommendations

This change also adds a new line to the sample config to match a pending
upstream change.

Change-Id: I9427c17a5be80de175ada8eb3d29fd99ab2b12bc
Merge "Enable support for secure cookies"