Allow to use any S3 compatible store

[johanneswuerbach]

deis-store is great, but it feels less production ready then deis (crashed twice yesterday and bringing it back up took 2 hours) and when the store is not running, everything is down. Most deis components are using deis-store already via the S3 compatible gateway so I would make sense from my point-of-view to have this option configurable.

Side note: I'm already using S3 as registry storage and RDS as database, but an unavailable deis-store still results in a not bootable registry, which means non of your apps will start / restart.
Issue: https://github.com/deis/deis/blob/master/registry/bin/boot#L48

[carmstrong]

but it feels less production ready then deis

store landed in Deis v0.13, and has been quite reliable. All of the store issues we're tracking currently seem to be due to a missed consideration with our upgrade process, which can result in a bumpy ride. We're tracking this in #2300 and I'm looking into it now.

You can't swap out store completely due to the controller and logger using CephFS to share log data. We chose Ceph because it provided us a backing store with both an S3-compatible blob store as well as a shared filesystem.

[bacongobbler]

oops. Since most of the conversation has been ongoing in #2812 let's keep it rolling over there.