[testlib] call init_config once the config has been properly bootstraped This should be done right after repository instantiation and as such involves giving CWTC.init_config as a callback to the test database handler (which will itself gives it to the repository initialization function). This unfortunatly requires to pass the init_config method to the server.init_repository function because it has to be called after the config has been initialized (which is done in Repository.__init__) but before the migration handler is instantiated (which will call 'server_maintainance' hook, hence may require the proper config). Another way to fix this would be to change the initialization sequence, but this is another story. Closes #3749378

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