Add API schema for v2.1/v3 availability_zone extension

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: I4a7bd2ef4df322eb072730bbf7da2766f02cb398
Merge "Add API schema for v2.1/v3 availability_zone extension"