Add value only output formattter

It may be useful to have a value only output formatter to perform operationgs like:
   TOKEN=$(openstack token create -c id -f value)
Rather than:
   eval $(openstack token create -c id -f shell)
   TOKEN="${id}"

Change-Id: I7bda3cc1a1d154a05e8e31eed564dae1e82066d6
Closes-Bug: #1292578