Fix ironic compute_driver name
Fix ironic compute_driver name

This commit fixes breaking change [0].
Long driver names like 'nova.virt.libvirt.libvirtDriver' are
no longer available and 'libvirt.libvirtDriver' should be used
instead.

Reference:
[0] https://review.openstack.org/309504

Change-Id: I27a1b75b921c7401bc8614caadfd1e09e7dd5d65
Closes-Bug: 1574990
(cherry picked from commit e3a04dd857d8e6f8afc83a20673fedd79936d19b)

samgoon/devstack,samgoon/devstack