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
Reformat coding standards to bring line lengths under 80 characters
Take into account Jason Frey's feedback - indent bullet list items that
wrap to multiple lines to align with 1st letter of next line
Do not indent inside code blocks