Qualify ghost user attribute in case ldap_group_nesting_level is set to 0
    
    When the sssd is set to not resolve nested groups with RFC2307bis, then
    the LDAP provider takes a different path. We didn't qualify the ghost
    users in this case.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/3236
    
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>
    (cherry picked from commit 538a7f1dd8339b90e0cfc64e7919a34d1d5c10d3)