Add a new optional WEBSSO_KEYSTONE_URL property to facilitate WEBSSO
deployments where network segmentation is used per security requirement.
In this case, the controllers are not reachable from public network.
Therefore, user's browser will not be able to reach OPENSTACK_KEYSTONE_URL
if it is set to the internal endpoint.
If WEBSSO_KEYSTONE_URL is set, it will be used instead of
OPENSTACK_KEYSTONE_URL.
Change-Id: I05ea4227aa4c2cb0a73015ed7fd29cf1a96e696a
Closes-bug: #1544703
a53f012fa1