sensible default for secure_proxy_ssl_header
sensible default for secure_proxy_ssl_header

there is only one sensible default for secure_proxy_ssl_header,
so let's use it, one less step for deployers to configure.

Change-Id: I0cee5d6051b2c91bc87dc7eabcec57dd4852184c
Closes-Bug: 1528258

ilay09/keystone,himanshu-setia/keystone,dims/keystone,openstack/keystone,rajalokan/keystone,cernops/keystone,ilay09/keystone,mahak/keystone,openstack/keystone,himanshu-setia/keystone,mahak/keystone,dims/keystone,klmitch/keystone,openstack/keystone,rajalokan/keystone,ilay09/keystone,cernops/keystone,klmitch/keystone,mahak/keystone,rajalokan/keystone