Add native WaitConditionHandle resource

Adds a native OS::HeatWaitConditionHandle resource, which works
in a similar way to the CFN compatible one, but with a few changes
to make it simpler to use:
- The data passed is validated less strictly, so we tolerate missing keys
  for any of the data (we just fill in default values)
- A signal passed with no data is assumed to mean success
- There are two convenience attributes which provide a string representing
  the necessary curl call to send the signal (to the native ReST API)

This allows a similarly simple signalling mechanism to the CFN compatible
Handle resource, but with no dependency on the ec2tokens keystone extension
or heat-api-cfn service.

Some usage examples here:
  - https://review.openstack.org/106424

blueprint: native-waitcondition
Change-Id: Ie9b5aeb13bfab5fba55c1a49d1572e0777864b29
Merge "Add native WaitConditionHandle resource"