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.
We want to use ```ipa``` over ```freeipa```, to avoid ruling out other IPA-compatible technologies. So

```
Enable configuration for external authentication via IPA
```

for the description and either keep it ```enable_ipa``` or ```ipa_authentication```.