util: restructure tracevol.py function to get_subvol_group()
util: restructure tracevol.py function to get_subvol_group()

get_subvol_group() returns empty string if subvolumeGroup is not defined,
changed it to return "csi" as default subvolumeGroup.

Signed-off-by: Mudit Agarwal <e5356c34e3b3b8ef2f9750bb431df6fcd5e2fa6e@redhat.com>

ceph/ceph-csi,ceph/ceph-csi,ceph/ceph-csi,ceph/ceph-csi