There are two different kinds of admission control checks, hence two
locations.

* From tbswap, easy checks are made (like, too many nodes of a real
  type), or trying to the use robot lab when its closed.

* From assign_wrapper.in. Because of virtual types, and because assign
  is the ultimate descision maker, we need to pass into assign per
  physical type limits, and let it deal with how virtual types map to
  physical types, and whether it exceeds the limits. So, when called
  from assign_wrapper, libadminctrl returns a hash of types and the
  counts the current experiment is limited to. That is put into the
  ptop file where assign deals with it.

If admission control goes sour, then just turn off the sitevar for it:
'swap/use_admission_control'