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 get the following error when I try ```apache::mod { 'authnz_pam': }```:

```
(File[/etc/httpd/conf.d] => Class[Apache] => Package[ruby193-rubygem-passenger-native] => Class[Foreman::Install] => Class[Foreman::Config] => Apache::Mod[authnz_pam] => File[authnz_pam.load] => File[/etc/httpd/conf.d])
```