Add API schema for v2.1 "resize 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 "resize
a server" extensions.

Partially implements blueprint v2-on-v3-api

Change-Id: I3651da8c1d23ee1b5d4710b1b323ecf9337ca70d
Merge "Add API schema for v2.1 "resize a server" API"