Change LOG.warn to LOG.warning

Python 3 deprecated the logger.warn method, see:
https://docs.python.org/3/library/logging.html#logging.warning
so we prefer to use warning to avoid DeprecationWarning.

Change-Id: I932a82be1e0f88bb1e333a9a7c3fc5bf87fa9e79
Closes-Bug: #1530742

gnocchi/cli.py

@@ -138,7 +138,7 @@ def metricd():
         _metricd_cleanup(workers)
         sys.exit(0)
     except Exception:
-        LOG.warn("exiting", exc_info=True)
+        LOG.warning("exiting", exc_info=True)
         _metricd_cleanup(workers)
         sys.exit(1)