SettingsParser default value should not pass to atype function

[rickmak]

The atype function should only run if the value is provided by the duser

• Skygear Server Date/Version: v1.3.0
• Application Platform: py-skygear
• [ ] Is this a regression?
• [ ] Attached logs, screenshots

Expected Results

The atype function should not accept the default value, and it should safely assume the pass in value is a string from envvar or command line.

Actual Results

The atype function will accept the default value, making the developer need to handle type and return. resulting following code:

def get_provider_enabled(enabled):
    if isinstance(enabled, bool):
        return enabled

    try:
        return strtobool(enabled)
    except ValueError:
        return False

The isinstance check should not be required.