Add env var to dump config graph

Make this a bit easier during debugging.  Add env var and some
developer instructions.

Change-Id: I34978ddb47d6642dfa22cae0f4c0543c0ba5475f

tox.ini

@@ -10,6 +10,8 @@ deps= -r{toxinidir}/requirements.txt
       -r{toxinidir}/test-requirements.txt
 commands=
     python setup.py test --slowest --testr-args='{posargs}'
+passenv=
+    DUMP_CONFIG_GRAPH
 
 [testenv:pep8]
 commands =