WIP: Upload build logs to S3

[manics]

Playing around with S3 logs again (#1156). This time uploading repo2docker logs from the BinderHub side instead of the repo2docker side.

[minrk]

Following comments in jupyterhub/repo2docker#967 , if we do it on the binderhub side, I think a sidecar container in the build pod might be a better way to go because we don't have to think about the consequences of binder pod replicas and restarts. A build can have 0, 1, or 2 (or more) RequestHandlers and binder pods watching it, whereas doing it in repo2docker (or in the build pod at least) should mean the lifecycle and everything is more naturally aligned.

Cases we would need to handle if we did at the level in this PR:

• 0 watchers if no requests are currently connected
• 2 (or more) watchers if multiple requests to watch a build of the same image are routed to different binder replicas. There's still one build pod, but multiple pods watching the build logs. This may even happen with multiple requests to one pod, I'd need to refresh my memory, though.

[manics]

Thanks... after reading the feedback on jupyterhub/repo2docker#967 I think r2d is the best (and easiest) place for it, let's continue on jupyterhub/repo2docker#967