installutils: add DM password validator
    
    Add a validator that checks whether provided Directory Manager
    is valid by attempting to connect to LDAP.
    
    Related https://pagure.io/freeipa/issue/6892
    
    Signed-off-by: Tomas Krizek <tkrizek@redhat.com>
    Reviewed-By: Martin Basti <mbasti@redhat.com>
    Reviewed-By: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>