The balance_pools command come with some subtle and undocumented
usage restrictions.  Most importantly:
  * Some DUTs not in Stierlin are in pools that shouldn't be
    changed using balance_pools.
  * New pools for general testing should be discouraged, since
    failures in those pools can't be managed by our usual processes.

Both of these restrictions can be dealt with by changing balance_pools
to require that both pools it operates on be managed pools.

That is, the balance_pools command operates on two pools:  A main
pool and a spare pool (the spare pool is commonly unspecified, defaulting
to "pool:suites").  The command should require that both of those
pools be in the list of pools specified as
    site_utils.suite_scheduler.constants.Pools.MANAGED_POOLS