Allow plugin selection when running //:broker from bazel

scripts/bazel/rabbitmq-run.sh

@@ -71,9 +71,9 @@ RABBITMQ_LOG=${RABBITMQ_LOG:='debug,+color'}
 export RABBITMQ_LOG
 
 if [ -z ${LEAVE_PLUGINS_DISABLED+x} ]; then
-    RABBITMQ_ENABLED_PLUGINS=ALL
+    RABBITMQ_ENABLED_PLUGINS=${RABBITMQ_ENABLED_PLUGINS:=ALL}
 else
-    RABBITMQ_ENABLED_PLUGINS=
+    RABBITMQ_ENABLED_PLUGINS=${RABBITMQ_ENABLED_PLUGINS:=}
 fi
 
 mkdir -p ${TEST_TMPDIR}