Extend API schema for "create a server" extensions

Some v2.1/v3 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 "create
a server" extensions. As a sample of API schema extensions, this
patch adds the schema of keypair extension.

Partially implements blueprint v3-api-schema

Change-Id: I972d648546b1fcb37c6ad5e12a0a49ce810a35f9
Merge "Extend API schema for "create a server" extensions"