Framework for objectify API requests

Senlin has provided a rich set of APIs in its version 1.0 release. Moving forward, more APIs will be introduced and existing APIs may get changed. While these changes are inevitable, we do want to ensure that all API parameters are properly validated even in the face of version bumps.

The goals of this validation (framework) are:

- Validate every API parameter;
- Unify the message format and HTTP status code if the causes are the same. For example, "... is too short.", "... is not an integer".
- Cleanup existing ad-hoc parameter validation code;
- Lay foundation for future version changes.

Since all API request bodies are supposed to be encoded in JSON format, we will use JSONSchema validation for this job.

The validation will happen after a request body is deserialized, so it has a potential to support XML parameter encoding.

This work will translate all REST requests into versioned objects and then make use of the schema dumping functionality of versioned objects for parameter validation.