Devstack has some little known variables for running nova
with the fake compute driver and running several nova-compute
services on a single host, which can be useful for testing
move operations and scale testing of controller services like
nova-api and nova-scheduler.
This adds documentation about the fake virt driver and related
variables and scaling considerations when using them.
Change-Id: Ic89d463d0f3f180b323edd6e2c8ff0404638ef07