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.
Nothing explicit, but something I'd guess something like:
```puppet
if $foreman::enable_ipa and ! $foreman::passenger {
  fail("Foreman must be configured with passenger to enable IPA authentication")
}
```