Blockserver: Ensure test_active_config_holder uses IOC objects in the same way that the real code does

As a developer I would like my tests to be testing the code as it is actually run in production. In the test_active_config_holder.py tests IOC objects are created to be added to the config. It looks like the real code uses dictionaries instead of these objects.

Acceptance Criteria:
* In ioc.py remove get and __getitem__ (note that these aren't used in the real codebase)
* Running the active config tests should still pass