Move list_apps() get_remote part in a separate ContainerRegistry object
This is a serious rework of how we list image tags from registries.
The MR is not perfect though:
- the new unit tests might be better moved in kaboxer test
- we should have some hello-kbx images on gitlab and docker hub, just for tests
- we need more testing, especially for the GitLab API
- we should optimize the GitLab API part, based on the assumption that the last elem of the image path is the app name
Closes: #39 (closed) #47 (closed)