Change neutron CI dashboard to a week view

The Neutron CI team, the primary customer for the Neutron Failure Rate
dashboard [1] meets once a week to review CI status.  It would be useful
if we did not need to change the Grafana view to a one-week view each
time, so a "time" stanza is added in accordance with the grafyaml
documentation [2].

[1] http://grafana.openstack.org/dashboard/db/neutron-failure-rate
[2] https://docs.openstack.org/infra/grafyaml/

Change-Id: I20853dc71cbcc280e7039a1404d17c81ec172c5a

grafana/neutron.yaml

@@ -1,5 +1,8 @@
 dashboard:
   title: Neutron Failure Rate
+  time:
+      from: "now-7d"
+      to: "now"
   rows:
     - title: Description
       height: 100px