Improve test suite

[lkubb]

What does this PR do?

• Drops long EOL Vault versions from test suite, adds 1.14.8 as static reference point (it's the last version with the previous license)
• Uses upstream containers instead of SaltStack mirror
• Raises pytest output verbosity
• removes hard dependency on Docker as container orchestrator
• tries to autodetect a rootless Podman socket if not running in CI to be able to run the functional/integration tests

What issues does this PR fix or reference?

Fixes: #51

Previous Behavior

• tests run also against laughably outdated Vault versions
• trying to run the tests with Podman does not work

New Behavior

• tests against Vault latest and 14.1.8
• Podman rootless works for local testing

Merge requirements satisfied?

• Docs
• Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• Tests written/updated

Commits signed with GPG?

Yes