Add descriptions for by default admin only field.
Add descriptions for by default admin only field.

Some fields such as OS-EXT-SRV-ATTR:host by default only
returned when the caller is admin, we need tell API user
about that.

Change-Id: I6021def80ffeceb30bd1b5c2ccfb5576662ceaac
Partial-Bug: #1515222

openstack/api-site,dreamhost/api-site,Tesora/tesora-api-site,dreamhost/api-site,Tesora/tesora-api-site,dreamhost/api-site,dreamhost/api-site,dreamhost/api-site,Tesora/tesora-api-site,Tesora/tesora-api-site,openstack/api-site,openstack/api-site,Tesora/tesora-api-site,dreamhost/api-site,openstack/api-site,Tesora/tesora-api-site,Tesora/tesora-api-site,dreamhost/api-site