Check key definition in TOSCA template and report incorrectly defined keys

Currently if the key is not properly defined, the translator will ignore it rather than report an error or warning. For example, a server instance should have a property named 'num_cpus', if it is spelled as 'num_cpu', an exception of missing required field will raise. If it is an optional property like 'description', the template can get parsed but the property is missing. So it's needed to validate the key definition as heat template parser does to see if it meets the TOSCA specification.