Add API schema for v2.1/v3 create_backup 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: Ie8a1ce6d0d7b90fb73555981fa974e9029344d3b
Merge "Add API schema for v2.1/v3 create_backup API"