When directly piping in the source of a systemd unit file, cdist
errored out. This is due to not propagating the stdin file to the
underlying __config_file type (which already contains code for this
case, but this has to be duplicated in __systemd_unit).
The following example thus works:
__systemd_unit test.service --source - <<EOF
[Service]
ExecStart=/bin/sleep 3600
EOF