gmake run-broker, gmake start-cluster: use a very small default plugin list

Currently both commands start a node with all plugins.
That's never the user's intent and it can result
in confusing plugin conflicts that would have never
happened in any real world plugin configuration.

Author: michaelklishin

Makefile

@@ -66,16 +66,17 @@ ifdef COMMUNITY_PLUGINS
 DEPS += $(RABBITMQ_COMMUNITY)
 endif
 
+# Plugins to enable for `gmake run-broker`, `gmake start-cluster`, etc.
+# To override:
+#     gmake run-broker ENABLED_PLUGINS="rabbitmq_management rabbitmq_stream"
+#     gmake start-cluster NODES=3 ENABLED_PLUGINS="rabbitmq_management rabbitmq_stream rabbitmq_stream_management"
+ENABLED_PLUGINS ?= rabbitmq_management
+RABBITMQ_ENABLED_PLUGINS ?= $(call comma_list,$(ENABLED_PLUGINS))
+export RABBITMQ_ENABLED_PLUGINS
+
 include erlang.mk
 include mk/github-actions.mk
 
-# If PLUGINS was set when we use run-broker we want to
-# fill in the enabled plugins list. PLUGINS is a more
-# natural space-separated list.
-ifdef PLUGINS
-RABBITMQ_ENABLED_PLUGINS ?= $(call comma_list,$(PLUGINS))
-endif
-
 # --------------------------------------------------------------------
 # Distribution - common variables and generic functions.
 # --------------------------------------------------------------------