Allow the S3 credentials for a domain to come from a AWS Token Service

[bmbouter]

On our deployment we store S3 bucket credentials in another service and they can't be handed to Pulp directly in all cases.

What can be done is those other services can hand Pulp a token which can be used to act on behalf of a user who does have access to those services. To do that they use the AWS Token Service.

How could Pulp use a token like that to configure django-storages?

How to configure Pulp to fetch the tokens is not something I have a good idea on yet, but let's assume we already have the token. Then Pulp could:

1. Here's an example of another project that uses STS to configure Boto.
2. The django-storages docs say we can use a Boto session to configure django-storages.