Added 'combined' to container_id options

[jwiederh]

General information

This small changes adds the 'combined' option for container_id in the docker.cfg.
Output for the docker agent_based plugins does not change.
Replicates #774 because I accidentally closed the previous pull request.

Proposed changes

The corresponding config would look like this:

[DOCKER]
container_id: combined
and the result would be piggyback hosts with the scheme <hostname_of_docker_host>_<container_name>.

Ideas Portal suggestion: https://ideas.checkmk.com/suggestions/532086/include-hostname-in-container-name

Forum discussion: https://forum.checkmk.com/t/hostname-translation-for-piggyback/51039/8