Check for explicitly defined Marathon port first.

Previously, we did the check too late resulting in the traefik.port
label not being effective.

The change comes with additional refactorings in production and tests.

testhelpers/helpers.go

@@ -0,0 +1,6 @@
+package testhelpers
+
+// Intp returns a pointer to the given integer value.
+func Intp(i int) *int {
+	return &i
+}