Add API schema for v3 admin_password 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: I97a412aa23db75e3d892a113733e732eeb8008c1
Merge "Add API schema for v3 admin_password API"