Add API schema for v3 keypairs 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: I484fc2807721b5bfe0f1a11c3811b99a13d920aa
Merge "Add API schema for v3 keypairs API"