Ssh clean up

[jayjb]

Proposed changes

This commit introduces a few changes:

• ssh key paths if you would like to generate your own and supply the path
• ssh code clean up
• config code cleanup enabling checks for ports and config variables.

Types of changes

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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Documentation Update

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• Lint and unit tests pass locally with my changes (if applicable)
• I have run pre-commit (pre-commit in the repo)
• I have added necessary documentation (if appropriate)

[benjamin-thinkst]

Looks good. Some suggestions that may suit this change.

[benjamin-thinkst]

Do we want to add the length constraint here too.
see https://github.com/thinkst/opencanary/pull/309/files#diff-5f3e79391c8c84f0500a2fbfaab887c2661e9e86cb9f26cdeb9cb6da69a0a308L194

[jayjb]

        if key == "ssh.version" and len(val) > 253:
            raise ConfigException(key, "SSH version string too long (%s..)" % val[:5])

We already have this which results in a nicer error message too. So going to leave it as is