Allow non-Amazon S3 backends

[jiri]

There's currently a fair share of S3-compatible object stores, for example MinIO. It would be nice if PopSpace didn't require using Amazon's services to prevent further centralization of the web.

[jiri]

Brief consultation with MinIO docs shows that the aws-sdk package used by PopSpace should be usable with MinIO no problem, so this really should be as easy as adding env vars and using them in the right places :)

[a-type]

I think it's a great idea! I'm not very happy about being tied to S3 either. If you're familiar with the details of how to enable that, I'd happily review a PR with the required changes. Otherwise I'll take a look at what's necessary when I have a bit of time.

[jiri]

Sounds great! I'll try to take a look as well, it looks like it should be just a few lines of changes and I've already got a running MinIO instance to test against 👍