Fix cloudkitty exception handling from gnocchiclient

Probably due six removal, exception handling from gnocchiclient
in cloudkitty is not working as expected.

This patch fixing this issue.

Bug described in storyboard:
https://storyboard.openstack.org/#!/story/2008985

Story: 2008985
Task: 42635
Change-Id: I387fee79d277daa4c739297f1734dad1eaf4cc94
(cherry picked from commit 302f3c8)

Author: keuko

cloudkitty/collector/gnocchi.py

@@ -319,7 +319,7 @@ def _fetch_metric(self, metric_name, start, end,
             # FIXME(peschk_l): gnocchiclient seems to be raising a BadRequest
             # when it should be raising MetricNotFound
             if isinstance(e, gexceptions.BadRequest):
-                if 'Metrics not found' not in e.args[0]:
+                if 'Metrics not found' not in e.message["cause"]:
                     raise
             LOG.warning('[{scope}] Skipping this metric for the '
                         'current cycle.'.format(scope=project_id, err=e))