Add API schema for v3 flavor_access 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 nova-api-validation-fw

Change-Id: Ia3b4fa951c2f3867685ea6a619a39ac47dc4c3e6
Merge "Add API schema for v3 flavor_access API"