[CI] Fix multiple gate issue

1. Use OVS main branch

git checkout of OVS master branch causes an error [1].
OVS deleted its master branch and replaced it with main branch.

2. Cap setuptools <71.0.0

py38 jobs (on ubuntu-focal) started to fail (see the relevant github
issue [1]) due to recent virtualenv release (20.26.4; which bundles
setuptools).

setuptools is bundled in virtualenv, so it has to be capped via the
virtualenv package. tox also needed to be capped (<4) as gate uses
tox 3.28.0, but with capping virtualenv we pull in latest tox as well,
which would cause other errors.

3. Remove lower-constraints job

lower-constraints jobs are not supported anymore and are removed from
most of the branches, so let's remove here as well to unblock the gate.

[1] error: pathspec 'master' did not match any file(s) known to git
[2] pypa/setuptools#4483

Conflicts:
  zuul.d/project.yaml

NOTE(elod.illes): conflict was due to branch specific job template

Change-Id: I010e145f4c5079a80b72c5229dfacc4d6896373e
(cherry picked from commit f2d7949)
(cherry picked from commit 1a49de7)

Author: Elod Illes

tools/setup-ovs.sh

@@ -1,6 +1,6 @@
 #!/bin/bash -xe
 
-OVS_BRANCH=${OVS_BRANCH:-master}
+OVS_BRANCH=${OVS_BRANCH:-main}
 OVN_BRANCH=${OVN_BRANCH:-main}
 
 function use_new_ovn_repository {

tox.ini

@@ -3,6 +3,12 @@ minversion = 3.1.1
 envlist = py38,py36,pep8
 skipsdist = True
 ignore_basepython_conflict = True
+# Cap setuptools via virtualenv to prevent compatibility issue with yoga
+# branch's upper constraint of 'packaging' package (21.3).
+requires =
+  virtualenv<20.26.4
+  tox<4
+  setuptools<71.0.0
 
 [testenv]
 basepython = python3

zuul.d/project.yaml

@@ -1,7 +1,6 @@
 - project:
     templates:
       - check-requirements
-      - openstack-lower-constraints-jobs
       - openstack-python3-wallaby-jobs
       - publish-openstack-docs-pti
     check: