Add API schema for v3 agents 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 nova-api-validation-fw

Change-Id: Iae1bff0e853e1dc597ebdca07efeb477b105469e
Merge "Add API schema for v3 agents API"