Add section on Keystone Token Binding

Havana added this feature, here are the docs.

Change-Id: I9c8c785f3eed91cc688f49e7dda7228cc1ff5216
Closes-Bug: 1196775
Add information on KVM guest agent feature

Havana added this feature, so documenting in the
configuration reference.

Change-Id: I5e5ae475e24854e48df59d985342bc0c1c52050e
Closes-Bug: 1201652
Merge "Add information on KVM guest agent feature"
Merge "Add section on Keystone Token Binding"