Nova fails to start with error: "nova.virt.driver ImportError: Class LibvirtDriver cannot be found"

Issue

Nova fails to start

# systemctl   start  openstack-nova-compute.service
Job for openstack-nova-compute.service failed. See 'systemctl status openstack-nova-compute.service' and 'journalctl -xn' for details.
# systemctl status openstack-nova-compute.service
openstack-nova-compute.service - OpenStack Nova Compute Server
   Loaded: loaded (/usr/lib/systemd/system/openstack-nova-compute.service; disabled)
   Active: failed (Result: start-limit) since Mon 2015-10-12 12:22:51 UTC; 2s ago
  Process: 30946 ExecStart=/usr/bin/nova-compute (code=exited, status=1/FAILURE)
 Main PID: 30946 (code=exited, status=1/FAILURE)

Oct 12 12:22:51 compute0.local systemd[1]: Failed to start OpenStack Nova Compute Server.
Oct 12 12:22:51 compute0.local systemd[1]: Unit openstack-nova-compute.service entered failed state.
Oct 12 12:22:51 compute0.local systemd[1]: openstack-nova-compute.service holdoff time over, scheduling restart.
Oct 12 12:22:51 compute0.local systemd[1]: Stopping OpenStack Nova Compute Server...
Oct 12 12:22:51 compute0.local systemd[1]: Starting OpenStack Nova Compute Server...
Oct 12 12:22:51 compute0.local systemd[1]: openstack-nova-compute.service start request repeated too quickly, refusing to start.
Oct 12 12:22:51 compute0.local systemd[1]: Failed to start OpenStack Nova Compute Server.
Oct 12 12:22:51 compute0.local systemd[1]: Unit openstack-nova-compute.service entered failed state.

But at the same time libvirtd daemon is up and running on the same server:

# systemctl status libvirtd 
libvirtd.service - Virtualization daemon
   Loaded: loaded (/usr/lib/systemd/system/libvirtd.service; enabled)
   Active: active (running) since Mon 2015-10-12 12:14:41 UTC; 11min ago
 Main PID: 18831 (libvirtd)
   CGroup: /system.slice/libvirtd.service
           └─18831 /usr/sbin/libvirtd --listen
Oct 12 12:14:41 compute0.local systemd[1]: Starting Virtualization daemon...
Oct 12 12:14:41 compute0.local systemd[1]: Started Virtualization daemon.
Oct 12 12:14:41 compute0.local libvirtd[18831]: libvirt version: 1.1.1, package: 29.el7_0.7 (Red Hat, Inc. <