Only install plugin system-wide if INSTALL_TEMPEST is set

Similarly to main tempest installation check, we should only install
plugin on master branches (where INSTALL_TEMPEST is True), and not on
stable branches

Also drop all-plugin use for tox_envlist in stadium jobs that were using
it, it is deprecated and breaks with this new variable

Change-Id: Ia7238c2044053d77b559de544c22909c50e1a11f
Co-Authored-By: Ghanshyam Mann <gmann@ghanshyammann.com>
Closes-Bug: #1873776

.zuul.yaml

@@ -1213,7 +1213,6 @@
       - openstack/tempest
     vars:
       tempest_test_regex: ^neutron_tempest_plugin\.sfc
-      tox_envlist: all-plugin
       devstack_plugins:
         networking-sfc: https://opendev.org/openstack/networking-sfc
         neutron-tempest-plugin: https://opendev.org/openstack/neutron-tempest-plugin
@@ -1276,7 +1275,6 @@
       - openstack/tempest
     vars:
       tempest_test_regex: ^neutron_tempest_plugin\.fwaas
-      tox_envlist: all-plugin
       devstack_plugins:
         neutron-fwaas: https://opendev.org/openstack/neutron-fwaas.git
         neutron-tempest-plugin: https://opendev.org/openstack/neutron-tempest-plugin.git
@@ -1335,7 +1333,6 @@
       - openstack/tempest
     vars:
       tempest_test_regex: ^neutron_tempest_plugin\.vpnaas
-      tox_envlist: all-plugin
       devstack_plugins:
         neutron-vpnaas: https://opendev.org/openstack/neutron-vpnaas.git
         neutron-tempest-plugin: https://opendev.org/openstack/neutron-tempest-plugin.git

devstack/plugin.sh

@@ -12,8 +12,10 @@ function install_neutron_tempest_plugin {
 if [[ "$1" == "stack" ]]; then
     case "$2" in
         install)
+            if [[ "$INSTALL_TEMPEST" == "True" ]]; then
                 echo_summary "Installing neutron-tempest-plugin"
                 install_neutron_tempest_plugin
+            fi
             ;;
         test-config)
             echo_summary "Configuring neutron-tempest-plugin tempest options"