Hint for user that ssh config is not correct

[nicolas-kuechler]

Currently, we don't have a check to see whether ssh configuration is done correctly (forward agent configured, can connect to a host with the private key, etc.).

Maybe beforehand, we could check whether base ssh config is set with something like this: https://stackoverflow.com/a/38305248

Later after setting up the environment, we should have some checks with a good fail message.

Should work for euler and aws.