LUSetClusterParams: improve volume group removal

Currently LUSetClusterParams will remove the volume group if the vg_name
field passed in is not true, but not None. Setting the target volume
group to False or the empty string, though, is a bad idea because it's
not a boolean value, and at cluster init we set it to None if
--no-lvm-storage is passed. With this fix we handle '' (or any other
non-None false value) as the "unset" value, but actually store None in
the config.

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