Adding Documentation for Enabling External Authentication

Adding ManageIQ/Guides for installing and enabling
external authentication via IPA.
PR Review Changes to the External Authentication Guide.
Updating REST API Reference to mention Request Tasks

- For completeness and reflecting the /api/request_tasks collection.
Merge pull request #24 from abellotti/rest_api_request_tasks

Updating REST API Reference to mention Request Tasks
Merge pull request #21 from abellotti/external_auth

Adding Documentation for Enabling External Authentication