Start actinia worker automatically on VM - VM start

[mmacata]

See #396 for context

To have an automated option, actinia should be able to start the worker - either via terraform (and possible usage of terraformer) or direct API (e.g. Hetzner API).
This might be developed not in actinia-core but in separate "actinia-job-starter" lib.
This lib should be open for different types where the worker can run in the future: VM, kubernetes, HPC, ... with initial focus on VM.

• prepare_actinia: VM starts, VM is installed (via ansible / terraform / cloud init / cloud config?)