Fix Telegraf retention policy not found

New version of influxdb use "autogen" as default retention policy name.
Please see https://github.com/influxdata/influxdb/issues/3733 for more
info.

Change-Id: I8aeb47f60b3aeb022e0cd7aaac630d7cad5b0099
Closes-Bug: #1673914

ansible/roles/telegraf/templates/telegraf.conf.j2

@@ -14,7 +14,7 @@
 [[outputs.influxdb]]
   urls = [{% for host in groups['influxdb'] %}"{{ influxdb_proto }}://{{ hostvars[host]['ansible_' + hostvars[host]['api_interface']]['ipv4']['address']}}:{{ influxdb_http_port }}"{% if not loop.last %},{% endif %}{% endfor %}]
   database = "telegraf" # required
-  retention_policy = "default"
+  retention_policy = "autogen"
   write_consistency = "any"
   timeout = "5s"
 [[inputs.cpu]]