Validate configured -alertmanager.web.external-url and fail if ends with /

[pracucci]

What this PR does:
Earlier this week I was testing the alertmanager in the local setup and I've learned the hard way that ruler->alertmanger notification doesn't work in the local dev setup and that if you configure -alertmanager.web.external-url with an ending / the routing doesn't work as expected.

This PR fixes the local dev env and adds a validation rule on -alertmanager.web.external-url.

Which issue(s) this PR fixes:
N/A

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[pstibrany]

I've learned the hard way that ruler->alertmanger notification doesn't work in the local dev setup and that if you configure -alertmanager.web.external-url with an ending /

Instead of erroring out, can we fix the problem with /? Either by removing it from the path, or using path.Join when appending. Why does having / in external-url fail in the first place?

[pracucci]

Why does having / in external-url fail in the first place?

I don't have time to investigate it (I consider it low priority). I see two options: accept the validation as a signal of a misconfiguration or I can roll it back and we just keep the fix for the dev env.

[pstibrany]

I see two options: accept the validation as a signal of a misconfiguration or I can roll it back and we just keep the fix for the dev env.

Validation is better than rollback. Approved. We can improve it when there is more time.