LDAP: Search for original DN during auth if it's missing
    
    Resolves: https://fedorahosted.org/sssd/ticket/2077
    
    If during the LDAP authentication we find out that the originalDN to
    bind as is missing (because the ID module is not LDAP based), we can try
    to look up the user from LDAP without saving him just in order to
    receive the originalDN.