Source code refactoring. Extracted common functionality in cpp_helpers.c...

...c and python/python_generator.cc
Put this first print statement back to PrintServiceClass() and PrintServiceStub(). The result should be like:
void Generator::PrintServiceClass(...) {
  printer->Print("$class_name$ = ..."
                        "...."
                        "class_name", descriptor.name());
  printer->Indent();
  PrintDescriptorKeyAndModuleName(descriptor);
  printer->Print("))\n\n");
  printer->Outdent();
}