Add API schema for v2.1/v3 scheduler_hints 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: I7e54db52e55727f090128cc393a28043e1085636