use import_object_ns for compute_driver loading
use import_object_ns for compute_driver loading

Part of bp:virt-driver-cleanup

Convert to using import_object_ns which allows us
to import specific virt drivers as 'libvirt.LibvirtDriver'
instead of 'nova.virt.libvirt.libvirtDriver'.

Update testing to ensure this works with both short
names and long names (which need not be in the nova.virt
namespace).

Change-Id: I5eee3389c7719d5f361532b0eddaa249233283a5

n0ano/gantt,n0ano/gantt