improvement(images): drop supervisor from scylla SCT image

[dimakr]

Drops usage of supervisor in scylla-sct docker image and introduces a wrapper around docker-py library APIs, to perform direct service managent tasks in the container.

Testing

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PR pre-checks (self review)

• I added the relevant backport labels
• I didn't leave commented-out/debugging code

Reminders

• Add New configuration option and document them (in sdcm/sct_config.py)
• Add unit tests to cover my changes (under unit-test/ folder)
• Update the Readme/doc folder relevant to this change (if needed)

[dimakr]

@fruch This is a draft. Can you take a look and suggest if the direction is OK and we can continue with it? (a run of artifacts-docker-test with this change - https://argus.scylladb.com/tests/scylla-cluster-tests/45752905-3c3e-4ab4-bda9-621ec5f220b6)

For now this change manages the processes directly, but nothing is dropped from the scylla-sct image (supervisor is just ignored).
Couple points to think about/discuss are:

• as there is no scylladb image without supervisor, I made a local copy of docker-entrypoint.py. In this local copy the supervisor daemon is not started, but instead we start standalone processes that we need (e.g. sshd, node-exporter, etc.). Then in SCT when the scylla-sct image is built, e.g. during artifacts-dockerf-test, we copy the local version of docker-entrypoint.py into image.
  Once the supervisor is not used/started in the scylladb docker image itself, this workaround can be dropped.
• I added abstractions/wrappers for docker container operations and service management operations. Although the docker-py lib itself provides clear APIs, I wrapped few APIs we are interested in with additional logic.
  Should we keep it, or maybe it is complicating the thing unnecessarily?