convert py35 jobs to py3
convert py35 jobs to py3

Update the jobs that were explicitly using python 3.5 and have them
just use python 3 without specifying a minor version.

Change-Id: I38734f2956a30c01ff51e2d74a9c248e4304d08b

openstack/heat,noironetworks/heat,noironetworks/heat,openstack/heat