the metadata filed should allow additionalproperty to be backward compatible

[yangcao77]

Describe the bug
The current devfile schema is backward incompatible due to the additionalproperty in metadata field has been set to false.
It would cause error while using the latest schema to parse an old 2.0 devfile
For example: devfile from odo registry

schema validation failed due to the following error:

invalid devfile schema. errors :
- metadata: Additional property alpha.build-dockerfile is not allowed
- metadata: Additional property alpha.deployment-manifest is not allowed

To Reproduce
Steps to reproduce the behavior:
use the latest schema to parse devfile from odo registry

Expected behavior
should be able to pass the schema validation, even the extra fields will not be unmarshal/marshal

[elsony]

@yangcao77 Is there any remaining work for this given that PR #234 has been merged? Can we close this?

[yangcao77]

Yes. This one can be closed.