Horizon: Support Strict-Transport-Security header
Horizon: Support Strict-Transport-Security header

This allows operators to enable HTTP Strict-Transport-Security (HSTS)
for Horizon endpoint, to enforce usage of SSL.

Change-Id: I20c2aee6af03af96a4a2022681eabba75a6acf00

openstack/puppet-tripleo,openstack/puppet-tripleo,openstack/puppet-tripleo