Adding Documentation for Enabling External Authentication - Depends on External auth3 PR# 32

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


Avoid indenting inside of a code fence because copying it will get the indent as well.  Unless of course it is *supposed* to be indented.