Initial JSONSchema validation

- errors are now returned as tracebacks, not HTML
- added schema checking based on the handler type and validator
- content_json decorator rewritten completely
- common JSON types
- creation of nodes with non-number ID is now forbidden according to scheme

Implements: blueprint nailgun-api-requests-responses-validation

Change-Id: Icf49bb534ccb19872402b66532ef44418bd66a53
Merge "Initial JSONSchema validation"