Add Fake virt driver support

nova fake virt driver can be used to simulate running arbitrarily large
number of VMs.  When the fake virt driver is selected, all other arbitrary
limits are disabled (quotas, scheduler limitations) as well. The fake
virt driver can be selected by adding the following line to your localrc

      VIRT_DRIVER=fake

This setting can be used to do simulated large scale (as defined by
number of VMs) testing (such as boot 500 VMs at once) inside a single
devstack instance.

Change-Id: I000c3cba107cd91bef11c4d5325e1ce7308faa6c
Merge "Add Fake virt driver support"