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.
So the comment could read something like
> Prior to RHEL 6.6, httpd_dbus_sssd is unavailable

That way we know that once CentOS 6.6 is out for a while, we can discuss removing it in favor of ```selboolean```. (Personally I think requiring RHEL 6.6+ for FreeIPA external authentication is perfectly valid.)