fixes #6445 - Configure external authentication via FreeIPA.

With these changes, ```foreman-installer --foreman-ipa-authentication=true``` will configure external authentication using IPA 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 IPA server.