Define new functional test tox env for placement gate to run

We have placement-nova-tox-functional-py38 job defined and run
on placement gate[1] to run the nova functional test excluding
api and notification _sample_tests, and db-related tests but that
job skip those tests via tox_extra_args which is not right way
to do as we currently facing error when tox_extra_args is included
in tox siblings task
- https://opendev.org/zuul/zuul-jobs/commit/c02c28a982da8d5a9e7b4ca38d30967f6cd1531d

- https://zuul.openstack.org/build/a8c186b2c7124856ae32477f10e2b9a4

Let's define a new tox env which can exclude the required test
in stestr command itself.

Conflicts:
    tox.ini

NOTE(melwitt): The conflict is because change
I672904e9bfb45a66a82331063c7d49c4bc0439df (Add functional-py39 testing)
is not in Victoria.

[1] https://opendev.org/openstack/placement/src/commit/bd5b19c00e1ab293fc157f4699bc4f4719731c25/.zuul.yaml#L83

Change-Id: I20d6339a5203aed058f432f68e2ec1af57030401
(cherry picked from commit 7b063e4)
(cherry picked from commit 64f5c1c)
(cherry picked from commit baf0d93)

Author: Ghanshyam Mann

tox.ini

@@ -126,6 +126,16 @@ deps = {[testenv:functional]deps}
 commands =
   {[testenv:functional]commands}
 
+[testenv:functional-without-sample-db-tests]
+description =
+  Run functional tests by excluding the API|Notification
+  sample tests and DB tests. This env is used in
+  placement-nova-tox-functional-py38 job which is defined and
+  run in placement.
+deps = {[testenv:functional]deps}
+commands =
+  stestr --test-path=./nova/tests/functional run --exclude-regex '((?:api|notification)_sample_tests|functional\.db\.)' {posargs}
+
 [testenv:api-samples]
 setenv =
   {[testenv]setenv}