Clean up configuration settings

Currently, the various configurations are registered in ./mistral/config.py. The configurations are registered when mistral.config is referenced. Given the way the code is written, PEP8 throws referenced but not used error if mistral.config is referenced but not called in the module. In various use cases, this is avoided by using importutils to import mistral.config (i.e. https://github.com/stackforge/mistral/blob/master/mistral/tests/unit/engine/test_transport.py#L34). This implementation should break down registration code in ./mistral/config.py into separate functions for api, engine, db, etc and move the registration closer to the module where the configuration is needed.