Add API schema for v2.1/v3 user_data extension

By defining the API schema, it is possible to separate the validation code
from the API method. The API method can be more simple.
In addition, a response of API validation error can be consistent for
the whole Nova API.

Partially implements blueprint v3-api-schema

Change-Id: If58e67a7fa0d256e307e565bb983c12ea131e439
Merge "Add API schema for v2.1/v3 user_data extension"