Feature configuration subsystem: extra_features and include

[tobixen]

Problem:

It's now possible to configure a server with "features" set to a string referencing a known server feature set.

Still it may be needed with local variations. It may be server versions, local settings etc causing the feature matrix to be different. It's also possible to specify special test configuration in the features - like, telling that it's OK to wipe all objects on the primary calendar.

Two ideas:

• Allow the features to include stuff. `features={'test-calendar': {'cleanup-regime': 'wipe-calendar'}, "include": "robur"}'
• Extra parameter "extra_features". `features="robur", extra_features={'cleanup-regime': 'wipe-calendar'}}

Of course, features=compatibility_hints.robur | {'test-calendar': {'cleanup-regime': 'wipe-calendar'}} works fine in Python, but it may be important to support various configuration file formats as well.