Use py38 instead of py37 jobs
Use py38 instead of py37 jobs

Python 3.8 is now our highest level supported python runtime.
Alternative runtime is py36. Change all py37 functional jobs to use py38
instead.

Change-Id: Ib37e9adb1921657f6a3155d9120de0bfa808d554

openstack/glance,openstack/glance,openstack/glance