fixes #6445 - Configure external authentication via FreeIPA.

With these changes, ```foreman-installer --foreman-enable-ipa=true``` will configure external authentication using FreeIPA per http://theforeman.org/manuals/1.5/index.html#5.7ExternalAuthentication, provided the machine where Foreman is being installer (or reconfigured) is IPA-enrolled and HTTP/foreman-fqdn service exists in the FreeIPA server.
While the packages are currently only available on these OSes, there is nothing Fedora or EL-specific about them -- anybody can and is welcome to add them to other distributions, or even Foreman can package and ship them. When they are not available, the installer will fail just fine when someone attempts ```--enable-ipa=true```. If we ```fail``` explicitly, you'll have to update the manifest any time it looks like new distribution is about to ship those packages.