pass pre-signed URLs to slugrunners to download slugs

[arschles]

continuing the discussion from #140

since slugrunners run in app namespaces, the object storage secret is not available to them and they can't download slugs. the current strategy to fix that is to copy the secrets into the app namespace so that slugrunners can mount and use them there.

IMO, we should keep the secrets in deis, generate pre-signed URLs to get the appropriate slug, and pass those URLs to new slugrunners.

cc/ @smothiki

[smothiki]

Does Minio support pre signed URL's.

[arschles]

@smothiki I haven't tested it yet. I've been assuming yes since all of their SDKs support them.

[helgi]

I'm bumping this out of beta. If this is an important thing for someone here then please add it back or create a milestone for latter releases

[arschles]

@helgi sounds good

[arschles]

Note that we'll need to think about how to support scale-up operations because the pre-signed URL that was used to start the first slugrunner will likely no longer be valid when the scale is requested.