TMSS-717: enforcing a minimum of one station in the list is too strict, sometimes an empty stationlist is ok.

TMSS-697: limit check for constraints change to when original doc was not None, hopefully fixing existing tests

TMSS-697: Add scheduling contraints to SU blueprint, copy from draft on creation, prevent update in non-schedulable state.

TMSS-715: Update check for default piggyback values propagation, only for objects in the 'creating' state

TMSS-666: removed set_subtask_state_following_allowed_transitions from subtasks.py. It was moved in TMSS-725 to test_utils.py