docs: clarify dependency on OpenSSH

[federicobozzini]

Changes

• topo actually depends on OpenSSH, not any generic ssh implementation. Let's make this explicit.

[th3james]

This is a bit misleading in its current form. It suggests that we've validated that OpenSSH is installed, but we've actually only validated that ssh is on the PATH. If we're doing this, we should probably test for OpenSSH, not just SSH

[federicobozzini]

This is a bit misleading in its current form. It suggests that we've validated that OpenSSH is installed, but we've actually only validated that ssh is on the PATH. If we're doing this, we should probably test for OpenSSH, not just SSH

I agree but this would misleading in the same way saying docker is OK is misleading when its version is not supported. At the moment we only check for binaries and ssh is the right binary for OpenSSH. We can replace the check with something better but I would treat it as a separate issue.