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
diff --git a/devstack/plugin.sh b/devstack/plugin.sh
index 25cfba6..7a46014 100644
--- a/devstack/plugin.sh
+++ b/devstack/plugin.sh
@@ -12,8 +12,10 @@
 if [[ "$1" == "stack" ]]; then
     case "$2" in
         install)
-            echo_summary "Installing neutron-tempest-plugin"
-            install_neutron_tempest_plugin
+            if [[ "$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"