Round Tripper of Fission Router: parameters be configurable (#713)

The Round Tripper parameters of timeout, keep alive time and Max retry
times is configured in router system environment.

And they are set to Round Tripper when router service initializing.

This setup new nested structure `tsRoundTripperParams` to transfer them.

router/functionHandler_test.go

@@ -22,6 +22,7 @@ import (
 	"net/http/httptest"
 	"net/url"
 	"testing"
+	"time"
 
 	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
 )
@@ -56,6 +57,12 @@ func TestFunctionProxying(t *testing.T) {
 
 	fh := &functionHandler{fmap: fmap,
 		function: fn,
+		tsRoundTripperParams: &tsRoundTripperParams{
+			timeout:         50 * time.Millisecond,
+			timeoutExponent: 2,
+			keepAlive:       30 * time.Second,
+			maxRetries:      10,
+		},
 	}
 	functionHandlerServer := httptest.NewServer(http.HandlerFunc(fh.handler))
 	fhURL := functionHandlerServer.URL