Carthage ssh_jump_host does not appear to support strings

It appeared that ssh_jump_host should be able to be a string but ssh_online attempts to call ssh_jump_host.ssh_online which is not an attribute of `str`

Start by locating the ssh_online implementation and tracing how it uses ssh_jump_host. Reproduce the issue with ssh_jump_host set to a string, then confirm the behavior works without attempting an unavailable str attribute and add coverage for that case if the project has an existing test location.

Written by the indexing model from the issue text.