libvirt unix_socket_group attribute

- libvirtd.conf template now suports unix_socket_group defined in
  a attribiutes.
- openstack-nova user shuld be member of libvirt group to be able to read
  from libvirt unix_socket. Implemented group resource in libvirt recipie.

Change-Id: I88370f2d7fd5147f3bb46a1411295868a8ea12ce
manage libvirt group using the chef group resource

also rename the libvirt group attribute name

Change-Id: Ie4304e6c26dafa3a1101c1ff8f90d0236f80914a
Merge "libvirt unix_socket_group attribute"
Merge "manage libvirt group using the chef group resource"