Refactor ssh key handling

[tomuben]

Background

In file spawn_test_database.py
ssh_key_file is re-used and overwritten several times: sometimes it gets a string assigned, sometimes a Path.

Originally posted by @tomuben in #426 (comment)

We should different variables and avoid re-assigning to the luigi parameter.

Acceptance Criteria

1. Type of SpawnTestDockerDatabase.ssh_key_file must be str:

ssh_key_file : str = luigi.OptionalParameter(None, significant=False) # type: ignore