Disable devstack image building for trove-scenario-mariadb-single CI job

Trove mainly uses trovestack script to trigger the CI tests, including
the guest agent image building, so the image building in devstack could
be skipped to decrease the testing duration.

Change-Id: I26fa0adc1a5e39612be15b75d37d22ea1cca3be1
Story: #2005391
Task: #30380

.zuul.yaml

@@ -193,6 +193,8 @@
     name: trove-scenario-mariadb-single
     parent: trove-devstack-base
     vars:
+      devstack_localrc:
+        TROVE_DISABLE_IMAGE_SETUP: true
       trove_test_datastore: mariadb
       trove_test_group: mariadb-supported-single
 

devstack/plugin.sh

@@ -357,9 +357,12 @@ function init_trove {
         --os-project-name ${ALT_TENANT_NAME}
 
     # build and upload sample Trove mysql instance if not set otherwise
-    if [[ ${TROVE_DISABLE_IMAGE_SETUP} != "TRUE" ]]; then
-        echo "Setup datastore image"
+    TROVE_DISABLE_IMAGE_SETUP=`echo ${TROVE_DISABLE_IMAGE_SETUP} | tr '[:upper:]' '[:lower:]'`
+    if [[ ${TROVE_DISABLE_IMAGE_SETUP} != "true" ]]; then
+        echo "Setup datastore image."
         _setup_minimal_image
+    else
+        echo "Skip datastore image building."
     fi
 
     # If no guest image is specified, skip remaining setup