make user and group names in vdsm udev rules configurable

In Ubuntu, the group name of qemu is not "qemu" but "kvm". This patch
makes the user and group names in the rule file configurable.

The patch is on going work of porting VDSM to Ubuntu
http://www.ovirt.org/VDSM_on_Ubuntu

Change-Id: If19ee4b8d7c47830e3be233e16f8d6e71b047a8c
Signed-off-by: Zhou Zheng Sheng <zhshzhou@linux.vnet.ibm.com>
Reviewed-on: http://gerrit.ovirt.org/13354
Reviewed-by: Alon Bar-Lev <alonbl@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>