`create_cluster_file.bash` has gone missing

Hi, it looks like 504f08a102a5df1927e7def5363e0d70c0fbbb04 removed the `create_cluster_file.bash` script as part of some refactoring.

The trouble is, that script is effectively part of the "public interface" of these images. The [documentation](https://github.com/apple/foundationdb/tree/master/packaging/docker#copying-into-other-images) still calls it out as a useful thing to copy into other images, and the sample apps in [Python](https://github.com/apple/foundationdb/blob/master/packaging/docker/samples/python/app/Dockerfile#L32) and [Go](https://github.com/apple/foundationdb/blob/master/packaging/docker/samples/golang/app/Dockerfile#L37) still follow this pattern.

Would you be OK restoring it? We've made an assumption that the file exists in the CI system for our Erlang bindings as well.

Start by reviewing commit 504f08a102a5df1927e7def5363e0d70c0fbbb04 and the packaging/docker documentation. Check the Python and Go sample Dockerfiles that still reference create_cluster_file.bash, then restore the script so those examples and the Erlang bindings' CI assumption remain valid.

Written by the indexing model from the issue text.