Plugins » Katello

Fixes #27418 - Support docker pull with pulp3

This supports docker pull with pulp3. Crane (pulp2) is still supported and the
registry URL is dynamically determined based on the support for Docker blobs.

Previously, the RegistryResource class would run code when loaded, setting up
the inherited class variables. This commit moves this and the new logic to a class
method. Since there are now backend calls (to check for pulp3) to set up the class
variables, it seems like this should be a more deliberate action. This also makes testing
much easier.

I'm open to other suggestions on how to handle the class inheritance and the class variables.

To test:
- Modify foreman/config/settings.plugins.d/katello.yaml, adding the pulp_registry_url
```yaml
:container_image_registry:
:crane_url: https://localhost:5000
:pulp_registry_url: http://localhost:24816
:registry_ca_cert_file: /etc/pki/katello/certs/katello-default-ca.crt
```
- Set up docker on pulp3 box. You will need an up-to-date box since https://github.com/pulp/pulp_docker/pull/391 was merged
- Sync repos, you can use https://cloud.docker.com/u/jomitsch/repository/docker/jomitsch/workflow-test for a small one.
- docker login $HOSTNAME
- docker pull $HOSTNAME/default_organization-docker-workflow_test (check name in docker repo details)
- docker pull should be successful

It's worth trying out some other repos or registries (like quay.io) and also we should ensure pulp2 functionality is not expected

There will be installer changes to come that will update both dev and prod's katello.yml, these will need to be merged along
with this PR

I also updated the rubocop_todo as it was throwing warnings for duplicate entries.