Credentials ConfigLoader updates

[ddybvig-perf]

Currently some tests from test-credentials.py are failing because there have been some changes to the ConfigFileLoader and ConfigLoader classes, namely that the load_root_config method is no longer static and the as_list method needs to be called from the UMAPIConfigLoader child class. The calls to these methods in the CredentialManager class need to be updated so that the tests will pass and we maintain functionality.