Druid datasource names should only be unique for a given cluster

[john-bodley]

Currently the Druid datasource names needs to be globally unique per the following uniqueness constraint however I sense it only needs to be unique for a given cluster., i.e., it's viable that multiple Druid clusters could have a datasource with the same name.

Make sure these boxes are checked before submitting your issue - thank you!

• I have checked the superset logs for python stacktraces and included it here as text if any
• I have reproduced the issue with at least the latest released version of superset
• I have checked the issue tracker for the same issue and I haven't found one similar

Superset version

0.19.1

Expected results

You should be able to create/update a Druid Datasource (druiddatasourcemodelview/add) where the datasource name is unique only within the confines of the cluster.

Actual results

Already exists.

Steps to reproduce

1. Ensure you have at least two Druid clusters defined.
2. Try creating a datasource in cluster B which has the same name as an existing datasource in cluster A.
3. The uniqueness constraint will prevent the datasource from being created.