Implement create-space-quota command.

[#76026862]
Allow negative numbers in quota definitions.

- Apparently we use -1 as a "sentinel" value in CC to represent
  unlimited quotas.  This seems to be the convention going forward, so
  we're permitting negative values across the board.

  Users beware.  We will no longer protect you from asking for
  craziness like -1500 instances.  We'll just pass that along to the CC
  and let it ride.

[Finishes #76026862]