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.
I'm having trouble reading this. I think some of these ```$::sssd_services``` should be written as ```$::sssd_services``` since they're facts. Also, the latter part seems to be filtering out ```ifp``` from ```$::sssd_services```, but then you merge it in again? Would you mind writing some tests for this?