Switching most ContentHost checkouts to container-based hosts

[JacobCallahan]

This change is relatively small but targets a much larger impact. Unless otherwise specified, ContentHost checkouts will use containers, when supported. This can be disabled at the fixture level by modifying the request params; the test level by marking the test with no_containers; or at runtime with the --no-containers option which will disable containers for all tests.

By default, any remote execution tests will not use containers.

A new config field has been added to container_repo.yaml to account for the new container host. This could additionally be used for image syncing. I additionally refactored the content_host.yaml structure to be more clear and support containers

[JacobCallahan]

Once we agree on the new config structure, I'll submit an MR for the changes in CI

[JacobCallahan]

@synkd that is the case for traditional containers. However, these images circumvent that behavior in subscription-manager and are seen as actual content hosts.