[wip] Rackspace client and Identity service

Refactor API key authentication into a Rackspace-specific extension of OpenStack identity v2. Along the way, bootstrap the Rackspace provider package with functions to create and authenticate clients.

## Progress

- [x] Create the Rackspace identity service as a delegating wrapper around the OpenStack identity v2 service.
- [x] Remove API key authentication from the OpenStack identity service. Move it into the Rackspace service.
  - [x] Refactor `token.Create`'s request body generation into a method on a wrapped `AuthOptions` struct. Use method overriding to interact with it from the rackspace/ provider call.
- [x] Write the `NewClient` and `AuthenticatedClient` functions to create a provider client that's authenticated through the Rackspace identity API.
- [x] Add delegation for the extensions service.
- [x] Add delegation for the tenants service.
- [x] Unit tests for:
 - [x] OpenStack endpoint location (since I separated it out and made it public).
 - [x] Rackspace client creation.
- [x] Acceptance tests for:
 - [x] Rackspace client creation.
 - [x] Listing Rackspace identity extensions.
 - [x] Listing Rackspace token tenants.
- [x] Add a method to override `createPage` in an existing `Pager` rather than just exporting the field.
- [x] Refactor copy-pasted test fixtures into a build-tagged *fixtures.go* file.
 - [x] tokens
 - [x] tenants
 - [x] extensions
- [x] Create `script/` scripts to run unit and acceptance tests properly.