gnt-cluster modify: fix --no-lvm-storage

Currently doing a gnt-cluster-modify --no-lvm-storage is silently
ignored, as it passes a None value in vg_name, which is the same as not
modifying that parameter. Explicitely set the passed value to '', so the
non-true not-None value can be evaluate to actually remove a volume
group.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>