[WIP] Adding Documentation for Enabling External Authentication - Depends on External auth3 pull request.

Adding ManageIQ/Guides for installing and enabling
external authentication via IPA.

Depends on the External auth3 Pull Request https://github.com/ManageIQ/manageiq/pull/32


When talking about the ManageIQ Appliance, Appliance should be capitalized.

User accounts will be automatically created on the Appliance
and relevant information imported from the IPA Server.