Add helmDefaults.repoForceUpdate and repositories[].forceUpdate configs.

[wi1dcard]

Add helmDefaults.repoForceUpdate and repositories[].forceUpdate in helmfile.yaml. With this PR, users can specify whether they'd like to enable --force-update globally, individually, or just leave it disabled (by default).

Related PRs or issues:

• Append --force-update for specific helm versions. #1494
• helmfile repos attempts to re-add already added repo #1489
• Fixing issue with idempotent repo add helm/helm#8777

[wi1dcard]

Found a problem here: helm seems not to update repos without the --force-update option, even though the URL specified in helmfile.yaml and existing helm repos are the same:

Adding repo stable https://kubernetes-charts.storage.googleapis.com/
"stable" already exists with the same configuration, skipping

Adding repo gitlab https://charts.gitlab.io
"gitlab" already exists with the same configuration, skipping

I don't see any good solution for now. Either we add the option --force-update (which could leave potential vulnerabilities), or don't add --force-update (which let helm just ignore adding or updating the repo if it already exists) is not the great option I think.

Probably we need to perform a helm repo update once helm repo adds are done, or otherwise we'd have to keep the --force-update option for now and apply it to Helm >=3.3.2 (this requires another hotfix PR, currently --force-update is only applied to Helm >=3.3.2 && <3.3.4, UPDATE: I've created another PR #1546 to fix the bug with Helm v3.3.4 first, please merge that PR then we can discuss more detail about how we should improve later on.)

Is there any better idea?

[wi1dcard]

This PR should be able to pick up and implement now as Helm has added helm repo update <repo_name>. I'll rebase it to the latest later and finish it as soon as I'm available.

[yxxhero]

see: #2148