Add API schema for v2.1 "rebuild a server" API

Some v2.1 APIs(create/update/rebuild server) contains the API
extension feature, it searches extension methods with its method
name and operates them automatically.
It is necessary to change the API schemas also automatically based
on the API extension loading condition, because the API extensions
add API parameters.

This patch adds API schema extensions which are implemented with the
same mechanism (stevedore library) as the API extension for "rebuild
a server" extensions.

Partially implements blueprint v3-api-schema

Change-Id: Ibfdd99966335d062cde4cc1d9ab3b63bc82ed0b6
Merge "Add API schema for v2.1 "rebuild a server" API"