Add API schema for v2.1/v3 multiple_create 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: Ibf0b6a5024f834ac9252933568d4416a8a5ba1fc
Merge "Add API schema for v2.1/v3 multiple_create extension"