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

After discussion with the Horizon developers, this change does not
include setting SESSION_COOKIE_HTTPONLY since that is the default in
settings.

(cherry picked from commit 1bf1c1e04c2ce84315d49a4bf5aa3438ee901719)
(cherry picked from commit bdac8c0f04a7c0727e600bb042a558ba7fd4844f)

Change-Id: I147807f2f5f7441548787f6730025955aad7529a