Use placement stable version for functional job

nova-tox-functional is py2 job and need placement as required project.
and started failing for incompatible py version:

ERROR: Package 'openstack-placement' requires a different Python: 2.7.17 not in '>=3.6'

- https://zuul.opendev.org/t/openstack/build/b460a8c59ad64e57b871aa3c00638b01/log/job-output.txt#782

Placement master is now py3 only which mean master version will stop
working on py2 env:
- I6f458fb60b5a33b5aa2ce3ab292862ab98eb4670

Current tox env mention the placement master link as deps, where need to use
the stable version for stable jobs. Making deps with version not the
fresh git clone like how other test-requirements.txt deps work.

Change-Id: I96bb9fcb55f2a4fc3aefa01db70b45740db166c8

tox.ini

@@ -60,17 +60,22 @@ commands =
 description =
   Run functional tests using python3.
 # As nova functional tests import the PlacementFixture from the placement
-# repository these tests are, by default, set up to run with latest master from
-# the placement repo. In the gate, Zuul will clone the latest master from
-# placement OR the version of placement the Depends-On in the commit message
-# suggests. If you want to run the test locally with an un-merged placement
-# change, modify this line locally to point to your dependency or pip install
-# placement into the appropriate tox virtualenv. We express the requirement
-# here instead of test-requirements because we do not want placement present
-# during unit tests.
+# repository these tests are, by default, set up to run with openstack-placement
+# from pypi. In the gate, Zuul will use the installed version of placement (stable
+# branch version on stable gate run) OR the version of placement the Depends-On in
+# the commit message suggests. If you want to run the tests with latest master from
+# the placement repo, modify the dep line to point at master, example:
+# deps =
+#   {[testenv]deps}
+#   git+https://opendev.org/openstack/placement#egg=openstack-placement
+# If you want to run the test locally with an un-merged placement change,
+# modify the dep line to point to your dependency or pip install placement
+# into the appropriate tox virtualenv.
+# NOTE: We express the requirement here instead of test-requirements
+# because we do not want placement present during unit tests.
 deps =
   {[testenv]deps}
-  git+https://opendev.org/openstack/placement#egg=openstack-placement
+  openstack-placement>=1.0.0
 commands =
 # NOTE(cdent): The group_regex describes how stestr will group tests into the
 # same process when running concurently. The following ensures that gabbi tests