heat engine : Add metadata validation to WaitConditionHandle

Validate metadata before updating the WaitConditionHandle resource
metadata, if we raise a ValueError in response to incorrectly
formatted metadata then the CFN API will map this to a
HeatInvalidParameterValueError, resulting in a descriptive error
response to the malformed request

Change-Id: I6fde27066e9e0eaee97b59a7fbbace09ca620b48
Signed-off-by: Steven Hardy <shardy@redhat.com>