Support Default Group for Users with LDAP authentication.

When Default Group for Users is enabled in Configure - Configuration - Authentication,
this default group can be used for all ldap users who use LDAP for authentication only,
for example, when an LDAP server does not have memberof attribute for groups.
    
https://bugzilla.redhat.com/show_bug.cgi?id=1060207

If you're not using any of the returns values, then don't assign anything.