Add API schema for v2.1 "create a server" 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 v3-api-schema

Change-Id: Ic17d2227909c9180ca86c9d578dad4c73ec57dfc
Merge "Add API schema for v2.1 "create a server" API"