Add API schema for v2.1/v3 flavor_manage 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: I92de34f6b9801b7ff9e4a9cc77baf2bf2bb36f6f
Merge "Add API schema for v2.1/v3 flavor_manage API"