state that x-subject-token is required
state that x-subject-token is required

the x-subject-token request header is required for head/delete/get
of the /auth/tokens API

Change-Id: I3a7f68c9e84d856a9c29a9e006922facb1bccdf0
Closes-Bug: #1527729

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