Add documentation for configuring IdP WebSSO
Add documentation for configuring IdP WebSSO

We recently added a new federation call to Keystone that will allow federated
authentication flows for a specific Identity Provider. This commit adds
documentation around configuring httpd modules for the newly added call and
horizon configuration.

Co-Authored-By: Lin Hua Cheng <os.lcheng@gmail.com>

Change-Id: Id334e979c951387b1d70da70fc5d4939a6e7d6a6
related: bp federation-idp-websso
Closes-Bug: 1491910
Related-Bug: 1491916

rajalokan/keystone,roopali8/keystone,rajalokan/keystone,ajayaa/keystone,cernops/keystone,maestro-hybrid-cloud/keystone,himanshu-setia/keystone,ilay09/keystone,rajalokan/keystone,mahak/keystone,maestro-hybrid-cloud/keystone,dims/keystone,openstack/keystone,cernops/keystone,ilay09/keystone,mahak/keystone,roopali8/keystone,dims/keystone,ilay09/keystone,klmitch/keystone,openstack/keystone,mahak/keystone,openstack/keystone,klmitch/keystone,himanshu-setia/keystone,ajayaa/keystone