[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 you wrap text indent the next line up the the first character of the same line.  It makes it easier to read in non-rendered mode.

e.g.

```
* Installing the required packages - [Installation](./external_auth/installation.md)
  * The packages required for enabling External Authentication are
    installed by default on the Appliance so these steps are intended
    to assist when building an appliance from scratch.
```