ClickHouse config improvements

[denisov-vlad]

1. added support to exclude databases from tables list

ClickHouse has local and distributed tables. Distributed table has the same structure as local but uses all hosts available in cluster to get data. We use special database for local tables which are unused in redash.

2. added support of multiple hosts

• simple load balancing for cluster with same dbs and tables structure
• if multiple hosts are passed it iterates over them until first response will be received
• use random order as option

[arikfr]

Hi,

(This is a template message, but I mean every word of it. Also you're welcome to reply)

Thank you for making this contribution. While we couldn't bring it to completion and merge, it's still very much appreciated. 🙇

In the past year the Redash code base gone under massive updates: on the backend we moved to Python 3 & RQ instead of Celery and on the frontend we replaced Angular with React. It's very likely this makes this PR irrelevant without significant changes. :-(

I'm closing this PR now. But if you're still interested in making it happen, let me know and I will reopen.

Thanks.