fix 409 exception regex
fix 409 exception regex

NamedMetricAlreadyExists does not catch the correct message.

Change-Id: I951fde3afa80b41643fa3b4af6f14e41e4063f0d

gnocchixyz/python-gnocchiclient,sileht/python-gnocchiclient,gnocchixyz/python-gnocchiclient