Add API schema for v2.1/v3 server_metadata 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: I4ef7d71f5d8eba13535f2c1a4aaf2b1f8d73a922
Merge "Add API schema for v2.1/v3 server_metadata API"