Adding ENABLE_IDENTITY_V2 on docs
Adding ENABLE_IDENTITY_V2 on docs

Adds the documentation of ENABLE_IDENTITY_V2 property on keystone
section.

Change-Id: If72325751c3bfd7f11f46c518c9fbf59f10a894f

group-policy/devstack,richliu/devstack,dtroyer/devstack,openstack-dev/devstack,dtroyer/devstack,group-policy/devstack,samgoon/devstack,bigswitch/devstack,Mahito/devstack,bigswitch/devstack,richliu/devstack,samgoon/devstack,bswartz/devstack,dirkmueller/devstack,Mahito/devstack,openstack-dev/devstack,nawawi/openstack,noironetworks/devstack,noironetworks/devstack,dirkmueller/devstack,nawawi/openstack,bswartz/devstack