Add 'listener.wss.external.tls_versions' option

etc/emq.conf

@@ -1185,6 +1185,13 @@ listener.wss.external.access.1 = allow all
 ## Value: Duration
 ## listener.wss.external.proxy_protocol_timeout = 3s
 
+## TLS versions only to protect from POODLE attack.
+##
+## See: listener.ssl.<name>.tls_versions
+##
+## Value: String, seperated by ','
+## listener.wss.external.tls_versions = tlsv1.2,tlsv1.1,tlsv1
+
 ## TLS Handshake timeout.
 ##
 ## See: listener.ssl.<name>.handshake_timeout