Add support for cluster-scoped init_scripts when creating clusters.

[adampicwell]

It looks like the proper way to specify cluster-scoped init_scripts is when creating clusters: https://docs.databricks.com/clusters/init-scripts.html

It looks like it was added in this PR #196 but only to edit_cluster. In any case, it was (inadvertently?) removed in PR #200.

Can we have this as a kwarg to create_cluster and make it available on the CLI as well?