Add API schema for v2.1/v3 flavors_extraspecs API

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: I71f89b59dd4e542d2befa971182cc587b6fec269
Merge "Add API schema for v2.1/v3 flavors_extraspecs API"