Add script to generate the official-images library manifest file

Usage:

```console
$ ./generate-stackbrew-library.sh > /path/to/official-images/library/maven
$ cd /path/to/official-images
$ # make a PR out of the changes \o/
```
Remove full JDK version variations
Merge pull request #1 from tianon/generate-stackbrew-library

Add script to generate the official-images library manifest file