RHEL-6/CentOS-6 is no longer supported, let's remove dependency on
libcgroup and code that enables/starts cgconfig service.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1602407Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>