Newly added ssh commands were missing the flag -i to pass an
identity file. This means that those commands use different
settings then the provisioners and their connection blocks
around them.
While adding this parameter, I decided it would be cleanest
to add local.ssh_args.
Setting private_key to null uses the local ssh-agent as a fallback for
authentication. Using the public_key instead of the private_key for
ssh -i lets the agent select the right identity if loaded. tested
with a yubikey
