RFE: Devstack plugin

Some functional tests require setting up a custom environment on which to run.

For example, federation tests require configuration of a service provider module in Apache, and federation with an external identity provider such as Shibboleth or another keystone (for keystone-to-keystone federation.)

Fortunately, Devstack includes a mechanism for allowing scripts to be plugged in so they can be called by stack.sh during the different phases of installation, making it ideal for preparing the custom environments we require.[1]

The previous attempt[2] at writing a Devstack plugin for keystone was overly confusing and difficult to test. To make reviewing and testing it easier, it will be broken up into smaller pieces. These will be slowly rolled out, incrementally adding the functionality required for the testing environment.

The purpose of this RFE is to have a reference describing the undertaken approach.