CI: Fix trunks enabled by default

Cardoe reported that networking-baremetal jobs were failing regardless
of the change. Upon investigation, we could see that n-g-s's devstack
plugin was automatically turning on the the test feature flag.

By default, this should be assumed to be false, and as such the
developer has moved to using a setting flag, I'll take a similar
approach with the the ngs devstack plugin so we don't inadvertently
turn on the feature flag for tempest.

see: https://review.opendev.org/c/openstack/ironic/+/940611
Change-Id: I7f80a33382e163478a65e6acb1f478f610f6b488

Author: juliakreger

devstack/plugin.sh

@@ -235,7 +235,9 @@ function ngs_configure_tempest {
     if [ $GENERIC_SWITCH_USER_MAX_SESSIONS -gt 0 ]; then
         iniset $TEMPEST_CONFIG ngs port_dlm_concurrency $(($GENERIC_SWITCH_USER_MAX_SESSIONS * 2))
     fi
-    iniset $TEMPEST_CONFIG baremetal_feature_enabled trunks_supported True
+    if [[ "${ML2_L3_PLUGIN:-}" =~ "trunk" ]]; then
+        iniset $TEMPEST_CONFIG baremetal_feature_enabled trunks_supported True
+    fi
 }
 
 # check for service enabled