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 believe if this can be simplified using stdlib. As I read it, you want to ensure some values are in there. One can simulate a set using [unique](https://github.com/puppetlabs/puppetlabs-stdlib#unique) and [union](https://github.com/puppetlabs/puppetlabs-stdlib#union). The sort is there so you get consistent output.

```puppet
# $input = strip(split($sssd_ldap_user_extra_attrs, ','))
$input = ['email:mail', 'x:y']
$defaults = ['email:mail', 'lastname:sn', 'firstname:givenname']
$output = sort(unique(union($input, $defaults)))
notice join($output, ', ')
```

Results in:
```
Notice: Scope(Class[main]): email:mail, firstname:givenname, lastname:sn, x:y
```