Add a config DB table

[knoxsp]

It is becoming increasingly desirable to have config data stored in the database instead of hydra.ini. This is because changing config now requires restarting the services, while a config DB table would see updates immediately.

[pmslavin]

This is also a prerequisite for the concept of a hydra-admin frontend which allows dynamic config changes.

There are a few features this should provide:

1. Authentication and authorisation: only permitted users can make changes. Some sets of config options may be configurable by only certain sets of users.
2. Validation: It must not be possible to specify a config value which results in abnormal operation, or to omit any config values required for normal operation.
3. Config sets: It should be possible to create named and versioned complete sets of configuration options which can be immediately switched between. These should be cryptograpically verified to ensure that only authentic and integral config sets can be loaded.
4. Extensibility: Future developments may require the introduction of new config options and the retirement of existing options. These changes should be performed programatically and without requiring alterations to the implementation of the config system. This can be supported by allowing the config api to define, modify, and delete config options in a structured way.

[pmslavin]

See also corresponding hydra-client changes in hydraplatform/hydra-client-python#19