Use low-memory-usage.yaml in tripleo jobs

low-memory-usage.yaml is generally used in TripleO standalone jobs now,
thus it is better to enable it in TripleO jobs in each puppet modules.

Change-Id: I40145696225bda8ab2b229148a5fbec9249df7e0

zuul.d/tripleo.yaml

@@ -48,6 +48,10 @@
       - ^setup.cfg$
       - ^setup.py$
       - ^tox.ini$
+    vars:
+      featureset_override:
+        standalone_environment_files:
+          - 'environments/low-memory-usage.yaml'
 
 - project-template:
     name: tripleo-puppet-standalone