Streamline ssh access to sessions via the CLI

[olevski]

See https://renku.discourse.group/t/error-cannot-start-session-via-the-notebook-service/870/10

Two main issues:

• When the CLI is used to access the session via ssh then only a single specific (to that renku deployment) key pair from ~/.ssh/renku is being used to log in, but we should simply try all key pairs in ~/.ssh and in ~/.ssh/renku.
• Revise/change the logic for how the CLI reports that a session supports SSH. This flag should be true/yes if the ssh daemon is running in the session. Not based on whether whoever is listing the session has access to it via ssh. From testing this feature out it seems that the SSH supported field starts out being set to false/no and when you successfully log in once with ssh then it shows yes.