This prevents a job reporting 'success' when the appropriate python
interpreter cannot be found, when actually it didn't run at all.
Also change the default envlist to use generic 'py3' instead of a
specific version which might not be present.

Also change zuul config so the python-cinderclient-functional-py36
job runs on centos-8-stream nodes, where py36 should be available.
And change bindep.txt to specify the correct package name for
centos-8.

Jeremy Stanley has given a more thorough explanation of why this
is a good change:
http://lists.openstack.org/pipermail/openstack-discuss/2020-May/014810.html

This isn't a theoretical issue.  If you look at recent
python-cinderclient-functional-py36 job results (for example,
[0]), you'll see that Zuul reported 'success', but on a closer look,
you'll see that no tests were run.

[0] https://zuul.opendev.org/t/openstack/build/1bfc80638086405f8b29905cdd6f71be/log/job-output.txt#25470

Change-Id: I2e2aa24e1592b66b287c84eda97b5079c40a36ec

Test jobs will look for a bindep.txt file in the repo to determine
which system packages need to be installed. If it does not find a
local one in the repo it will fall back to an OpenStack-wide bindep
that has many packages not needed for most projects. This adds a
limited list needed for this repo.

For more details, see:
http://lists.openstack.org/pipermail/openstack-dev/2016-August/101590.html

Change-Id: Ic7e00b6a34c0e46c678e5c10ecf0e76faa9913c8