Refactor: libpe_status: use "container agent" instead of "container type"

... since it's not really the type of the container itself (e.g., podman can
launch Docker containers).

This makes one visible change: displaying a bundle in status output now shows
"Container bundle set:" rather than "Docker container set:",
"podman container set:", etc. The specific container technology is probably not
very useful since clusters are likely to run just one technology, and
additionally it wasn't entirely accurate since again we don't really have the
type of the container itself.