Docs: Keystone AD/LDAP cleanup
Docs: Keystone AD/LDAP cleanup

This patch cleans up the Keystone AD/LDAP back end section a bit
and adds an additional example where the LDAP back end is used as
a read-only resource.

Change-Id: I9b75d941d7aff4dcafd8e8f486e406455721340d

mancdaz/openstack-ansible,Logan2211/openstack-ansible,bunchc/openstack-ansible,yanyao/newton,cfarquhar/openstack-ansible,bunchc/openstack-ansible,stackforge/os-ansible-deployment,weezer/openstack-ansible,cloudnull/os-ansible-deployment,BjoernT/os-ansible-deployment,os-cloud/os-ansible-deployment,weezer/openstack-ansible,robb-romans/openstack-ansible,openstack/openstack-ansible,yanyao/newton,mrda/openstack-ansible,major/os-ansible-deployment,major/os-ansible-deployment,BjoernT/os-ansible-deployment,mrda/openstack-ansible,robb-romans/openstack-ansible,cfarquhar/openstack-ansible,BjoernT/openstack-ansible,mrda/openstack-ansible,major/openstack-ansible,yanyao/openstack-ansible,BjoernT/os-ansible-deployment,openstack/openstack-ansible,yanyao/openstack-ansible,major/openstack-ansible,mancdaz/openstack-ansible,os-cloud/os-ansible-deployment,cloudnull/os-ansible-deployment,BjoernT/openstack-ansible,stackforge/os-ansible-deployment,Logan2211/openstack-ansible