Ensure race condition doesn't save invalid id no.

* the dumb_repmgr_id search looks for the highst id number in use
  but if multiple nodes are coming up, they will get the same
  number. The search saves this id and in case of a conflict it
  will never update and the standby clone cmd will fail every time

* using the default attribute will allow another search to occur
  in the event of a conflict and the new id will then work