Add support for running multiple n-cpus for performance testing

When using the fake VIRT_DRIVER, we can launch an arbitrary number of
instances, the next step for performance testing is running an arbitrary
number of n-cpus. This can be done with the following settings in your localrc

 VIRT_DRIVER=fake
 NUMBER_FAKE_NOVA_COMPUTE=50

This can be used for things like measuring load from n-cpu's periodic tasks.

Change-Id: I40e0f05374e2c10c48ea6b8816ef6e7609b3dc80
Merge "Add support for running multiple n-cpus for performance testing"