provide flexible mechanism(s) for non-dj.conf 'store' configuration

[ixcat]

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External store configuration is currently handled exclusively via dj.conf - This has at least some drawbacks:

1. Store configuration must be separately enabled for all users. This is cumbersome and error prone
2. Inadvertent commit of shared credentials can cause security problems, and in the case of AWS S3 can result in potentially disruptive forced credential rotation and account shutdown

Some various ideas:

• standardizing table based lookuo
• integration with web services
• environment variables etc.

Ideally, some flexible mechanism should be defined, with at least one concrete implementation solving the AWS S3 case.

Optionally, this could be extended to all authentication/configuration information in general