Correct a noisy DeprecationWarning
Correct a noisy DeprecationWarning

Let's drop this annoying thing:
/usr/lib/python3.6/site-packages/tripleoclient/workflows/plan_management.py:328: DeprecationWarning: The 'warn' method is deprecated, use 'warning' instead
  "Updating plan with passwords.".format(name))

Change-Id: Ia161b57b1442c2cc4bc83b1c7f94bfaf59548e2c

openstack/python-tripleoclient