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.
This change makes it so that SSH errors are shown, whereas QUIET hid
those as well. The main change to QUIET was to get rid of warnings,
not errors, so this fixes that.
