commands/ssh-config: fix for multiple private keys

since we merged multiple private keys, the :private_key_path item of
ssh_info now comes back as an array. the ssh-config command had not yet
been updated to handle this properly.

here we fix that oversight and add a few unit tests around the config
generation.
Merge pull request #2581 from phinze/ssh-config-support-multiple-keys

commands/ssh-config: fix for multiple private keys