Add a TODO on the VerifyCluster option

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>

tools/cluster-merge

@@ -613,6 +613,7 @@ class Merger(object):
                                 (result.fail_reason, result.output))
 
   # R0201: Method could be a function
+  # TODO: make this overridable, for some verify errors
   def _VerifyCluster(self): # pylint: disable-msg=R0201
     """Runs gnt-cluster verify to verify the health.