[CI] Replace deprecated regex and remove Centos8 and pin xena-last

Latest Zuul drops the following warnings:

  All regular expressions must conform to RE2 syntax, but an
  expression using the deprecated Perl-style syntax has been detected.
  Adjust the configuration to conform to RE2 syntax.

  The RE2 syntax error is: invalid perl operator: (?!

This patch replaces the 'irrelevant-files' to 'files' with explicitly
listing the pattern which files should be the tests run against.

Conflicts:
  .zuul.yaml

NOTE(elod.illes): parts of the Xena version of this patch were not
needed as they were not introduced in Wallaby yet.
Also, devstack-gate based base job is dropped as it is not used in
zuul.yaml at all and devstack-gate is retired and removed completely.

Change-Id: If287e800fb9ff428dbe6f9c4c046627f22afe3df
(cherry picked from commit 9b77bae)
(cherry picked from commit 8223e6a)
(cherry picked from commit 510a27b)
(cherry picked from commit 197b14d)
(cherry picked from commit ffc252e)
(cherry picked from commit 23fc1b9)
(cherry picked from commit 6fadd99)

Author: Elod Illes

.zuul.yaml

@@ -1,34 +1,6 @@
 # See https://docs.openstack.org/infra/manual/drivers.html#naming-with-zuul-v3
 # for job naming conventions.
 
-- job:
-    name: nova-dsvm-multinode-base
-    parent: legacy-dsvm-base-multinode
-    description: |
-      Base job for multinode nova devstack/tempest jobs.
-      Will setup firewall rules on all the nodes allowing them to talk to
-      each other.
-    timeout: 10800
-    required-projects:
-      - openstack/devstack-gate
-      - openstack/nova
-      - openstack/tempest
-    irrelevant-files: &dsvm-irrelevant-files
-      - ^api-.*$
-      - ^(test-|)requirements.txt$
-      - ^.*\.rst$
-      - ^.git.*$
-      - ^doc/.*$
-      - ^nova/hacking/.*$
-      - ^nova/locale/.*$
-      - ^nova/policies/.*$
-      - ^nova/tests/.*$
-      - ^nova/test.py$
-      - ^releasenotes/.*$
-      - ^setup.cfg$
-      - ^tools/.*$
-      - ^tox.ini$
-
 - job:
     name: nova-tox-functional-py38
     parent: openstack-tox-functional-py38
@@ -95,7 +67,21 @@
     description: |
       Run tempest live migration tests against local qcow2 ephemeral storage
       and shared LVM/iSCSI cinder volumes.
-    irrelevant-files: *dsvm-irrelevant-files
+    irrelevant-files: &dsvm-irrelevant-files
+      - ^api-.*$
+      - ^(test-|)requirements.txt$
+      - ^.*\.rst$
+      - ^.git.*$
+      - ^doc/.*$
+      - ^nova/hacking/.*$
+      - ^nova/locale/.*$
+      - ^nova/policies/.*$
+      - ^nova/tests/.*$
+      - ^nova/test.py$
+      - ^releasenotes/.*$
+      - ^setup.cfg$
+      - ^tools/.*$
+      - ^tox.ini$
     vars:
       tox_envlist: all
       tempest_test_regex: (^tempest\.api\.compute\.admin\.(test_live_migration|test_migration))
@@ -189,24 +175,11 @@
     parent: devstack-tempest
     description: |
       Run tempest compute API tests using LVM image backend. This only runs
-      against nova/virt/libvirt/* changes.
-    # Copy irrelevant-files from nova-dsvm-multinode-base and then exclude
-    # anything that is not in nova/virt/libvirt/* or nova/privsep/*.
-    irrelevant-files:
-      - ^(?!.zuul.yaml)(?!nova/virt/libvirt/)(?!nova/privsep/).*$
-      - ^api-.*$
-      - ^(test-|)requirements.txt$
-      - ^.*\.rst$
-      - ^.git.*$
-      - ^doc/.*$
-      - ^nova/hacking/.*$
-      - ^nova/locale/.*$
-      - ^nova/tests/.*$
-      - ^nova/test.py$
-      - ^releasenotes/.*$
-      - ^setup.cfg$
-      - ^tools/.*$
-      - ^tox.ini$
+      against nova/virt/libvirt/*, nova/privsep/* and .zuul.yaml changes.
+    files:
+      - ^nova/virt/libvirt/.*$
+      - ^nova/privsep/.*$
+      - .zuul.yaml
     vars:
       # We use the "all" environment for tempest_test_regex and
       # tempest_black_regex.
@@ -528,11 +501,12 @@
         - nova-ceph-multistore:
             irrelevant-files: *dsvm-irrelevant-files
         - neutron-tempest-linuxbridge:
-            irrelevant-files:
+            files:
               # NOTE(mriedem): This job has its own irrelevant-files section
               # so that we only run it on changes to networking and libvirt/vif
               # code; we don't need to run this on all changes.
-              - ^(?!nova/network/.*)(?!nova/virt/libvirt/vif.py).*$
+              - ^nova/network/.*$
+              - nova/virt/libvirt/vif.py
         - nova-live-migration
         - nova-lvm
         - nova-multi-cell
@@ -582,11 +556,12 @@
         - nova-ceph-multistore:
             irrelevant-files: *dsvm-irrelevant-files
         - neutron-tempest-linuxbridge:
-            irrelevant-files:
+            files:
               # NOTE(mriedem): This job has its own irrelevant-files section
               # so that we only run it on changes to networking and libvirt/vif
               # code; we don't need to run this on all changes.
-              - ^(?!nova/network/.*)(?!nova/virt/libvirt/vif.py).*$
+              - ^nova/network/.*$
+              - nova/virt/libvirt/vif.py
         - tempest-integrated-compute:
             irrelevant-files: *policies-irrelevant-files
         - nova-grenade-multinode: