[GH-326] [GH-327] Add tests to detect templating errors in more common situations and fix

[andrewodri]

Summary

This PR adds tests to help identify templating errors by running helm template silently, and failing on errors. This also allows sensible default values to be defined (e.g. ingress.enabled=true, presistence.enabled=true and autoscaling.enabled=true) for more comprehensive coverage.

I attempted to implement this by using the latest version of chart-testing, however, the configuration options don't appear to work, and it could not effectively identify errors without supplying new values.yaml files.

This PR also fixes ingress template errors in the mattermost-chaos-engine and mattermost-grafana chart recently introduced by #322.

Ticket Link

Fixes #326 and #327

[mattermod]

The files .circleci/config.yml, tests/e2e-kind.sh, tests/validate-charts.sh are in the blocklist for external contributors. Hence, these changes are not tested by the CI pipeline active until the build is re-triggered by a core committer or the PR is merged. Please be careful when reviewing it.
/cc @mattermost/core-security @mattermost/core-build-engineers

[mattermod]

Hello @andrewodri,

Thanks for your pull request! A Core Committer will review your pull request soon. For code contributions, you can learn more about the review process here.

[mattermod]

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