GUACAMOLE-600: Add telnet connection timeout client setting.

guacamole-ext/src/main/resources/org/apache/guacamole/protocols/telnet.json

@@ -12,6 +12,10 @@
                 {
                     "name"  : "port",
                     "type"  : "NUMERIC"
+                },
+                {
+                    "name"  : "timeout",
+                    "type"  : "NUMERIC"
                 }
             ]
         },