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.

Fixed in aa0cc28e46498170bb09e21d3bfc9e1a2364a286.

> Also, the latter part seems to be filtering out ifp from $::sssd_services, but then you merge it in again?

I'm not filtering it out of $::sssd_services, I'm filtering it out of the list with difference so that I don't add it if it's already there.

> Would you mind writing some tests for this?

Tests are beyond my ability. My hope was that @ares could add them if they are really necessary.