Runner: ssh wrapper forces ssh to use ssh_id when it is provided

What

Forces ssh to actually use the id file when it is given.

Why

When your local .ssh directory contains lots of identity files, the ssh connection can fail even if you provide the relevant identity with -i.

By default, ssh will iterate over the identity in .ssh and then, in addition, use the file you provided. After a threshold of unsuccessful attempts (6 in my case), connection fails, before trying the provided identity.

How

Whenever an ssh_id is passed, we set ssh option "IdentitiesOnly=yes", which prevent ssh to look for other identities than the one provided.

Manually testing the MR

Checklist

• Update CHANGES.md. No need to document changes to documentation and tests.
• Make sure all new values, types etc. are documented in .mli files and that the generated documentation looks ok.
• Add tests in test/, if relevant.