Docker based SSH server for integration tests

[pkittenis]

Testing

Migrate embedded OpenSSH server used for integration testing to docker container. The embedded server causes flaky tests from race conditions.

Migrating to docker will simplify the testing code and make tests more reliable, plus it allows the project to test default configuration of SSH server as used in distributions.

To test against the broadest platform, best to target latest Ubuntu LTS release.

[pkittenis]

Unlikely to be worked on, questionable value. Current tests are consistent now that paramiko clients have been removed.