Move S3 auth & cloudfront_signer config validation to init

[gerrod3]

Moved the validation in get_default_settings to be inside __init__ and check the final instantiated values, not just the values from settings. So even if the values in settings are incorrect, you can still create a S3 class by passing in the right values.

[gerrod3]

@jschneier Wdyt about this refactor?

[jschneier]

I almost pushed something like this myself. Can you make it explicit what situation you are solving for?

[gerrod3]

I want to make sure that if you are instantiating your own storage classes and providing every possible value for it, it shouldn't be blocked by what is configured in settings. In our project, Pulp, we have a multi-tenancy feature that allows users to create their own domains with their own custom storage backends separate from the default storage. [0] These domains must not use or be affected by the default settings.

[0] https://docs.pulpproject.org/pulpcore/workflows/domains-multi-tenancy.html

[terencehonles]

This change seems reasonable, but we were relying on cloudfront_signer=None which this change forces it to be truthy and a falsy value allowed disabling the signing but still serving via cloudfront. I fixed this in #1326