Add driver details in architecture doc
Add driver details in architecture doc

Added missing keystone driver details in architecture doc

Change-Id: Iac6d4008d72feeffa7f12ad1fa3b1d6c2cd634fe
Closes-Bug: #1546189

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