Adds explanation of the Keystone Assignments feature
Adds explanation of the Keystone Assignments feature

Adds explanation of Assignments feature, with configuration details
for enabling a SQL backend in Keystone.

Change-Id: Ic9ac68eada1730877ad8203df75d56920a6d35cc
Partial-Bug: #1202902

openstack/openstack-manuals,saeki-masaki/openstack-manuals,openstack/openstack-manuals,potsmaster/openstack-manuals,potsmaster/openstack-manuals,gongysh2004/openstack-manual,saeki-masaki/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,gongysh2004/openstack-manual,AlekhyaMallina-Vedams/openstack-manuals,Akasurde/openstack-manuals,openstack/openstack-manuals,kairoaraujo/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,openstack/openstack-manuals,Akasurde/openstack-manuals,kairoaraujo/openstack-manuals