Remove SupersetDB

[dervoeti]

Description

As discussed in #351:

• Removed .spec.clusterConfig.loadExamplesOnInit (loading examples is not supported anymore by the operator, instead we want to document how it can be done)
• Removed all the SupersetDB related code
• Moved database init commands from a separate Job to the command of each Superset Pod (We can move it into an initContainer, needs discussion and probably a separate issue for that)
• Added liveness and readiness probes (I used these values: https://github.com/apache/superset/blob/35eb66a322f7938f840778633a4aea11c7f24dce/helm/superset/values.yaml#L300-L317)
• Changed Pod Management Policy to OrderedReady, to make sure Pods start after another and the init commands don't run in parallel
• It's still possible that during an upgrade from e.g. 1.5.3 to 2.1.0 some Pods temporarily are on 1.5.3 while others are already on 2.1.0. I don't think that's a problem though.
• Wait for SupersetCluster instead of SupersetDB in DruidConnection

I tested various scenarios manually (multiple replicas, different upgrade paths, also the DruidConnection stuff), everything worked as expected.

Definition of Done Checklist

• Not all of these items are applicable to all PRs, the author should update this template to only leave the boxes in that are relevant
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Author

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1. Changes are OpenShift compatible
2. CRD changes approved
3. Helm chart can be installed and deployed operator works
4. Integration tests passed (for non trivial changes)

Reviewer

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1. Code contains useful comments
2. (Integration-)Test cases added
3. Documentation added or updated
4. Changelog updated
5. Cargo.toml only contains references to git tags (not specific commits or branches)

Acceptance

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1. Feature Tracker has been updated
2. Proper release label has been added

[fhennig]

Is there also a ticket somewhere for adding instructions how to install the demo data again?

[dervoeti]

Will create one tomorrow, we'll discuss this briefly in the Arch meeting.

[dervoeti]

There it is: #397

[maltesander]

Nice work, just minor things. Changelog is missing.

[maltesander]

LGTM!