Add CloudFront Support

[bmbouter]

CloudFront can "front" an S3 bucket making it geo-distributed. See the docs here: https://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/GettingStarted.SimpleDistribution.html

As a user, I should be able to configure AWS correctly, and then tell Pulp's Domain storage configuration whatever it needs to know to have users receive content from the CloudFront Distribution instead of S3. The entire domain would be GeoDistributed, not individual Repos.

[bmbouter]

My understanding is Pulp will interact with S3 is the same way. The only difference (that I can tell) is the Content App when directing users to S3 will instead direct traffic to the CloudFront distribution instead of S3. The Domain storage config in Pulp will need more info to perform that redirect. Also we need to make sure the signing features don't regress.

We'll also need docs.

We'll also need to determine how we'll test this part of the feature set.

[mdellweg]

We'll also need to determine how we'll test this part of the feature set.

Assuming that django-storages does the heavy lifting, and we only care about getting the configuration right, that is exactly what we should test.

https://django-storages.readthedocs.io/en/latest/backends/amazon-S3.html#cloudfront-signed-urls