Improve policy doc for supported scope info
Improve policy doc for supported scope info

Nova does not support all the scope provided by
keystone. It is better to mention the supported
scope and default roles.

Also adding a table to map the legacy rules with new
rules.

Change-Id: If7f025b3eaeda2df0cb1efd567b8fc60e274d09c

mahak/nova,mahak/nova,openstack/nova,openstack/nova,mahak/nova,openstack/nova