If there are no LVM based disk templates enabled in QA, reset the LV
name after the corresponding gnt-cluster modify tests. Otherwise
gnt-cluster verify would complain about the missing volume group.

Also make sure that no disk templates requiring LVM are ever enabled
without actually providing a LV name.
Signed-off-by: Thomas Thrainer <thomasth@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>