Convert WaitConditionHandle to use handle_signal

The metadata_update interface does pretty much the exact same thing
as handle_signal, with the disadvantage that it's not exposed via
the native ReST API, so convert WaitConditionHandle to use
handle_signal.

This has the following advantages:
- Provides a step towards native waitcondition resources
- Enables signalling existing waitcondition resources via
  heat resource-signal (e.g the native API)
- Moves towards a standardized model where handle_signal is the
  only supported plugin interface for asynchronous signalling of
  resources.

To avoid breaking any CD folks who may not update heat-engine and
heat-api-cfn at the same time, the existing resource interface is
maintained with a deprecation warning (how long to maintain this
is TBC).

Change-Id: I453a24f78095dfa5b3a5311651b5f1ac40aa8b3f
blueprint: native-waitcondition