Moving functional jobs to Victoria testing runtime

As per Victoria testing runtime[1], we need to tests py3.6,
py3.7, and py3.8.

- py3.7 is being tested with integration jobs.
- py3.6 and py3.8 are tested with unit test jobs.

Nova functional tests are testing py3.6 which can be moved to the latest
python available in this cycle which is 3.8. We do not need to run functional
or integration tests on each supported python version. Testing them on
the latest python version is enough. Coverage of all supported python version
is achieved via unit tests job.

[1] https://governance.openstack.org/tc/reference/runtimes/victoria.html

Change-Id: I1d6a2986fcb0435cfabdd104d202b65329909d2b

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 3.1.1
-envlist = py37,functional,pep8
+envlist = py38,functional,pep8
 # Automatic envs (pyXX) will only use the python version appropriate to that
 # env and ignore basepython inherited from [testenv] if we set
 # ignore_basepython_conflict.
@@ -102,6 +102,13 @@ deps = {[testenv:functional]deps}
 commands =
   {[testenv:functional]commands}
 
+[testenv:functional-py38]
+description =
+  Run functional tests using python3.8.
+deps = {[testenv:functional]deps}
+commands =
+  {[testenv:functional]commands}
+
 [testenv:api-samples]
 setenv =
   {[testenv]setenv}