Allow offline builds to choose included java versions

[metron2]

The offline buildpack is very hefty because it includes three JDKs. It would be nice if instead of hard coding the three version support users were able to specify which versions they wanted instead. I suggest having a variable that defaults to the three versions but can be changed by the user.

[dmikusa]

To be resolved in #957. If you are interested in taking an advance look at the solution, please take a look and add comments to the PR. We'll likely merge the PR in about a week unless there are objections/active discussions.