Fix marathon provider

The IP-Per-Task PR introduced a bug using the marathon application
port mapping. This port should be used only in the proxy server, the
downstream connection should be always made with the task port.

    This commit fix the regression and adds a unit test to prevent new
problems in this setup.

provider/marathon.go

@@ -496,11 +496,6 @@ func (provider *Marathon) getCircuitBreakerExpression(application marathon.Appli
 
 func processPorts(application marathon.Application, task marathon.Task) []int {
 
-	// First using application ports
-	if len(application.Ports) > 0 {
-		return application.Ports
-	}
-
 	// Using default port configuration
 	if task.Ports != nil && len(task.Ports) > 0 {
 		return task.Ports

provider/marathon_test.go

@@ -1077,6 +1077,19 @@ func TestMarathonGetPort(t *testing.T) {
 				Ports: []int{80, 443},
 			},
 			expected: "443",
+		}, {
+			applications: []marathon.Application{
+				{
+					ID:     "application-with-port",
+					Ports:  []int{9999},
+					Labels: &map[string]string{},
+				},
+			},
+			task: marathon.Task{
+				AppID: "application-with-port",
+				Ports: []int{7777},
+			},
+			expected: "7777",
 		},
 	}