Add API schema for v2.1 block_device_mapping 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 v2-on-v3-api

Change-Id: I6d53ef2e3dad5a4ac9b67a5838f6e07606a8372b
Merge "Add API schema for v2.1 block_device_mapping extension"