{chart,scripts}: move datasource test to helm chart

[paulfantom]

Description

Alternative approach to #520 which allows to use helm test command to test datasource connectivity. This method may also be used later on to test data ingestion (issue #324).

Re-enable datasource test, fixes #517
Fix promql datasource in grafana, fixes #519

Type of change

What type of changes does your code introduce to tobs? Put an x in the box that apply.

• CHANGE (fix or feature that would cause existing functionality to not work as expected)
• FEATURE (non-breaking change which adds functionality)
• BUGFIX (non-breaking change which fixes an issue)
• ENHANCEMENT (non-breaking change which improves existing functionality)
• NONE (if none of the other choices apply. Example, tooling, build system, CI, docs, etc.)

[onprem]

As helm test is run as part of ct install, I guess we can remove the GH action and just rely on the test to be present to verify datasources, which is nice.

Since the check-datasources.sh script got removed, let's cleanup the Makefile target as well.

[paulfantom]

This PR is now doing a bit more than I initially anticipated. In addition to what was primarily being fixed it also changes the following:

• CI helm values are limited only to necessary overrides
• additional lint and e2e make targets were added and now they are also used in CI

[onprem]

LGTM, let's just address the missing env variables.