Add API schema for v2.1/v3 cells 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: Ic400a31fe2c05e07785a2dc6c4fd864e684f4965
Merge "Add API schema for v2.1/v3 cells API"