Switch job definitions to use new "tempest_plugin" variable

This new variable was recently added to zuul in [1] to build
TEMPEST_PLUGINS list by combining plugin names with devstack
base path.

In old stable branches we still have to use old version with
TEMPEST_PLUGINS devstack option configured directly.

[1] https://review.openstack.org/#/c/639374/

Change-Id: I6bc2a3e752234d20890d64ceaefa0714cd05038f
Closes-Bug: #1822453

.zuul.yaml

@@ -79,11 +79,12 @@
         - dvr
       devstack_localrc:
         USE_PYTHON3: true
-        TEMPEST_PLUGINS: /opt/stack/neutron-tempest-plugin
         NETWORK_API_EXTENSIONS: "{{ (network_api_extensions_common + network_api_extensions_tempest) | join(',') }}"
       devstack_plugins:
         neutron: https://git.openstack.org/openstack/neutron.git
         neutron-tempest-plugin: https://git.openstack.org/openstack/neutron-tempest-plugin.git
+      tempest_plugins:
+        - neutron-tempest-plugin
       devstack_services:
         tls-proxy: false
         tempest: true
@@ -225,6 +226,7 @@
       devstack_localrc:
         USE_PYTHON3: false
         NETWORK_API_EXTENSIONS: "{{ (network_api_extensions_common + network_api_extensions_tempest) | join(',') }}"
+        TEMPEST_PLUGINS: /opt/stack/neutron-tempest-plugin
 
 - job:
     name: neutron-tempest-plugin-api-rocky
@@ -295,6 +297,7 @@
       devstack_localrc:
         USE_PYTHON3: false
         NETWORK_API_EXTENSIONS: "{{ (network_api_extensions_common + network_api_extensions_tempest) | join(',') }}"
+        TEMPEST_PLUGINS: /opt/stack/neutron-tempest-plugin
 
 - job:
     name: neutron-tempest-plugin-scenario
@@ -355,6 +358,7 @@
       devstack_localrc:
         USE_PYTHON3: false
         NETWORK_API_EXTENSIONS: "{{ network_api_extensions | join(',') }}"
+        TEMPEST_PLUGINS: /opt/stack/neutron-tempest-plugin
       devstack_local_conf:
         test-config:
           # NOTE: ignores linux bridge's trunk delete on bound port test
@@ -375,6 +379,7 @@
       devstack_localrc:
         USE_PYTHON3: false
         NETWORK_API_EXTENSIONS: "{{ network_api_extensions | join(',') }}"
+        TEMPEST_PLUGINS: /opt/stack/neutron-tempest-plugin
       devstack_local_conf:
         test-config:
           # NOTE: ignores linux bridge's trunk delete on bound port test
@@ -407,7 +412,6 @@
         - dvr
       devstack_localrc:
         USE_PYTHON3: true
-        TEMPEST_PLUGINS: /opt/stack/neutron-tempest-plugin
         NETWORK_API_EXTENSIONS: "{{ (network_api_extensions_common + network_api_extensions_dvr) | join(',') }}"
         PHYSICAL_NETWORK: default
         DOWNLOAD_DEFAULT_IMAGES: false
@@ -420,6 +424,8 @@
       devstack_plugins:
         neutron: https://git.openstack.org/openstack/neutron.git
         neutron-tempest-plugin: https://git.openstack.org/openstack/neutron-tempest-plugin.git
+      tempest_plugins:
+        - neutron-tempest-plugin
       devstack_services:
         tls-proxy: false
         tempest: true
@@ -522,6 +528,7 @@
       branch_override: stable/queens
       devstack_localrc:
         USE_PYTHON3: false
+        TEMPEST_PLUGINS: /opt/stack/neutron-tempest-plugin
 
 - job:
     name: neutron-tempest-plugin-dvr-multinode-scenario-rocky
@@ -532,6 +539,7 @@
       branch_override: stable/rocky
       devstack_localrc:
         USE_PYTHON3: false
+        TEMPEST_PLUGINS: /opt/stack/neutron-tempest-plugin
 
 - job:
     name: neutron-tempest-plugin-designate-scenario
@@ -544,7 +552,6 @@
     timeout: 3600
     vars:
       devstack_localrc:
-        TEMPEST_PLUGINS: '"/opt/stack/designate-tempest-plugin /opt/stack/neutron-tempest-plugin"'
         DESIGNATE_BACKEND_DRIVER: bind9
         DOWNLOAD_DEFAULT_IMAGES: false
         IMAGE_URLS: http://download.cirros-cloud.net/0.3.4/cirros-0.3.4-i386-disk.img,
@@ -556,6 +563,9 @@
       devstack_services:
         cinder: false
         designate: true
+      tempest_plugins:
+        - designate-tempest-plugin
+        - neutron-tempest-plugin
       tempest_test_regex: ^neutron_tempest_plugin\.scenario\.test_dns_integration
     irrelevant-files: *tempest-irrelevant-files
 
@@ -568,6 +578,7 @@
       branch_override: stable/queens
       devstack_localrc:
         USE_PYTHON3: false
+        TEMPEST_PLUGINS: '"/opt/stack/designate-tempest-plugin /opt/stack/neutron-tempest-plugin"'
 
 - job:
     name: neutron-tempest-plugin-designate-scenario-rocky
@@ -578,6 +589,7 @@
       branch_override: stable/rocky
       devstack_localrc:
         USE_PYTHON3: false
+        TEMPEST_PLUGINS: '"/opt/stack/designate-tempest-plugin /opt/stack/neutron-tempest-plugin"'
 
 - project-template:
     name: neutron-tempest-plugin-jobs