Fix constructor signature

[c-w]

All the subclasses take a configuration dictionary as constructor argument, so the interface definition should also mention it. In addition to improving documentation, this change will also make it safer to use reflection to look up and instantiate specific implementations of the interface since the constructor signature now is part of the official interface.

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[margaretmeehan]

Nice improvements. We should be more explicit about what env vars are required where throughout the code.