`get_mem_opts` Only adds memory options if none are specified in the other variables.

Also the order you have is wrong.  In Java, I believe "overriding" of args happens (they aren't immutable), so you want the exact inverse to that output i.e.:

```
-Dx=JavaOpts -Dy=123 -DxSbtOpts -Dx=ActOpts
```

But you may want to verify that.