feat(storage): add S3 region endpoint

[kingdonb]

Rebased #1 and shed the parts of the PR that were about glide (as we are using go modules now)

[kingdonb]

I'm aiming to make it possible to set up minio or digitalocean spaces with an endpoint in charts/workflow/values.yaml

There are a lot of half-finished PRs with something towards this change, I think ideally this should just hijack the "s3" section in values with an optional "endpoint" url that says to leave blank if using AWS. We might call this storage type "s3" too, if it will have downstream effects, or we could call it "s3compat" and only invoke the endpoint URL if s3compat is chosen.

I don't know if it's possible for true AWS s3 to specify an endpoint or what meaning or significance it would have, if the region was also set, I don't want to think about what it would mean for those values to be in conflict. But if there is some use case on AWS for custom endpoint settings, then I suppose they can choose "s3compat"