added prefix settings of the module and rearrange go test code

outputs/opentsdb/opentsdb_test.go

@@ -50,6 +50,7 @@ func TestWrite(t *testing.T) {
 	o := &OpenTSDB{
 		Host:   testutil.GetLocalHost(),
 		Port:   24242,
+		Prefix: "prefix.test.",
 	}
 
 	// Verify that we can connect to the OpenTSDB instance