In preparation for switching the base job to ubuntu-bionic which has
Python 3.6 by default and no Python 3.5, be explicit about the
nodeset for py35 jobs so they will continue to work. Also configure
the lower-constraints job to include the py36 bindep profile in
preparation for when it switches nodesets.
Change-Id: I560fb0ab43dd85bb03f13eff8bb0358f768c0e9b