Update Databricks user guide

[PatrickkZ]

Description

Update Databricks user guide, before, user need to download and upload packages to get BigDL env ready on Databricks, and config by this way, the ray worker still can not find the packages, so we use a init script to prepare the BigDL env, user do not need to download and upload packages, a single init script will be enough, and this can also fix the ray worker problem. The code in this tutorial is verified on Databricks.