This bug is created as a clone of upstream ticket:
https://fedorahosted.org/sssd/ticket/2009

If the IPA server mode is on and the SSSD is running on the IPA server, then the server's extdom plugin calls getpwnam_r to read info about trusted users from the AD server and return them to the clients that called the extended operation.

The SSSD returns the subdomain users fully-qualified, ie "user@domain" by default. The format of the fully qualified name is configurable.

However, the extdom plugin returns the user name without the domain component.

We have a couple of options:
 * when ipa_server_mode is on, return the usernames non-qualified.
 * when ipa_server_mode is on, warn if the full_name_format is set to a non-default value. That would prompt the admin to change the format if he changed it to something exotic.

I think the second option sounds better.