Add API schema for v2.1/v3 migrate_server 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: I9f1eea8471af0f86dcf472ed8a4fdb634dfdb260
Merge "Add API schema for v2.1/v3 migrate_server API"