medium: idmgmt: Fix id assignment and update regression tests (bnc#865292)

* Resolve the last few issues with id assignment (hopefully)
* Fix reversed order of fencing-level assignments. This wasn't wrong, it
  just didn't match the previous behavior.

The assigned ids are not quite the same as they used to be, so
update the regression tests accordingly. Hopefully this is not
a problem.
medium: constants: Rename cluster attribute to cluster-name (fate#316118)