Fix broken logstash UDP support (TykTechnologies#2141)

This PR disables `gas` reconnection functionality when `"logstash_transport": "udp"` because it doesn't support `udp`.
 
Also, we need to fix doc here: https://tyk.io/docs/log-data/#a-name-aggregated-logs-with-logstash-a-aggregated-logs-with-logstash. It says `logstash_transport: The Logstash transport to use, should be "tcp".` but we support `udp` too. And, it would be easier for user if we give a logstash input configuration there like:

```
input {
	tcp {	
		port => 5044
		codec => json
	}
}
```
Fixes TykTechnologies#2136

main.go

@@ -107,6 +107,7 @@ var (
 const (
 	defReadTimeout  = 120 * time.Second
 	defWriteTimeout = 120 * time.Second
+	appName         = "tyk-gateway"
 )
 
 func getApiSpec(apiID string) *APISpec {
@@ -767,12 +768,25 @@ func setupLogger() {
 	if config.Global().UseLogstash {
 		mainLog.Debug("Enabling Logstash support")
 
-		conn, err := gas.Dial(config.Global().LogstashTransport, config.Global().LogstashNetworkAddr)
-		if err != nil {
-			log.Errorf("Error making connection for logstash hook: %v", err)
+		var hook *logstashHook.Hook
+		var err error
+		var conn net.Conn
+		if config.Global().LogstashTransport == "udp" {
+			mainLog.Debug("Connecting to Logstash with udp")
+			hook, err = logstashHook.NewHook(config.Global().LogstashTransport,
+				config.Global().LogstashNetworkAddr,
+				appName)
+		} else {
+			mainLog.Debugf("Connecting to Logstash with %s", config.Global().LogstashTransport)
+			conn, err = gas.Dial(config.Global().LogstashTransport, config.Global().LogstashNetworkAddr)
+			if err == nil {
+				hook, err = logstashHook.NewHookWithConn(conn, appName)
+			}
 		}
-		hook, err := logstashHook.NewHookWithConn(conn, "tyk-gateway")
-		if err == nil {
+
+		if err != nil {
+			log.Errorf("Error making connection for logstash: %v", err)
+		} else {
 			log.Hooks.Add(hook)
 			rawLog.Hooks.Add(hook)
 			mainLog.Debug("Logstash hook active")