fix support for private images in SoftLayer

I think this method should be changed:

* Why does it receive a `Map.Entry` parameter? Change it to receive a String and a SoftwareDescription object, to make its signature more clean and meaningful.
* Also, returning `null` is evil. Avoid that. As this is a private method only used to populate values in a Set, just make it return void and refactor to something like `addOperatingSystemToSet`, and pass the Set as a parameter too. This way the method logic is self-contained and you're not propagating details such as the null value (which can introduce NPE issues) to the outside.