Add API schema for v2.1/v3 hosts 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
whole of Nova APIs.

Partially implements blueprint v3-api-schema

Change-Id: Ie252dce67f8e9d3292d8dad767b6113f7affcffd
Merge "Add API schema for v2.1/v3 hosts API"