Fix DTLS backward compatibility

Fix DTLS backward compatibility

mbedTLS expects the close_notify message as warning alert,
but tinyDTLS sends fatal alert

Change-Id: I91046d4eb23f6b7537abe0a3a2a2e2c6c2893f14
Signed-off-by: Dmitriy Zhuravlev <d.zhuravlev@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/13961
Reviewed-by: Kevin Kane <kkane@microsoft.com>
Reviewed-by: Oleksii Beketov <ol.beketov@samsung.com>
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Jongsung Lee <js126.lee@samsung.com>
Reviewed-by: Jongmin Choi <jminl.choi@samsung.com>
Reviewed-by: Chul Lee <chuls.lee@samsung.com>
Reviewed-by: Randeep Singh <randeep.s@samsung.com>