Upgrade su92l to 18.04
Cwltool removed Python 3.5 compatibility. Ubuntu 16.04 comes with Python 3.5.
We stopped building packages for 16.04, that release will no longer be supported in Arvados 2.2.
Upgrade su92l to 18.04 before we release Arvados 2.2. Be a bit careful: two nodes are managed via salt, the rest via puppet. See #17198.