Add API schema for v2.1/v3 config_drive 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: Ia0681941c5449051c90425348466bd7dc44c2e45
Merge "Add API schema for v2.1/v3 config_drive extension"