[Devstack] Add stack user to libvirt group
[Devstack] Add stack user to libvirt group

Ironic is needed to have libvirtd perms to manage VMs. When nova is
enabled all needed perms are granted in nova devstack plugin.
This patch ensures that stack user is added to libvirt group when nova
is not enabled.

Change-Id: If65002d775c9ff001f5a8e27ed1f6af1f8b4cf40

pshchelo/ironic,openstack/ironic,SauloAislan/ironic,openstack/ironic,SauloAislan/ironic,NaohiroTamura/ironic,pshchelo/ironic,NaohiroTamura/ironic