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
Return policy error, not generic error

When nova net-create and net-delete are prohibited by policy, we should
raise policy violation error (403) to the user instead of service
unavailable (503) error which is incorrect.

Closes-Bug: #1260905
Change-Id: I2edc0eb09f1dc192ba47184ed24fc49e4fcd4b79
Merge "Return policy error, not generic error"
Merge "Add API schema for v3 keypairs API"