Follow orchestra's result vocabulary split: payload variable, completion scope, outcome

#3613729: Split the overloaded result vocabulary into the outcome and the completion payload splits orchestra's overloaded result vocabulary into
the outcome (the resolved decision a flow condition routes on) and the
completion payload (whatever complete() was called with). The node
config keys and one engine method yoyaku uses move with it, so yoyaku has to
follow or its shipped example workflow stops binding its steps.

• result_variable becomes payload_variable and
  result_scope becomes completion_scope, in
  yoyaku_orchestra_example's yoyaku_booking workflow
  (both the node config and the BPMN XML it embeds) and in the kernel tests that
  build workflows inline.
• ProcessControlInterface::resumeWithResult() becomes
  resumeWithPayload(), called by
  BookingProcessCorrelator and one functional test; the
  correlator's own $result parameters become $payload
  to match, and its interface documentation with them.
• The example variable names that hold a scalar outcome are renamed for the
  same reason the upstream ones were: cancel_result,
  cancel_order_result, confirm_result,
  form_result, payment_result,
  release_result, settle_result and
  yoyaku_deposit_result become *_outcome.

Pre-release, so no update hooks: a site's saved workflows and any process
variable holding a payload under the old key are rewritten by hand.