Adds prometheus_scrape_interval

Grafana requires the scrape interval to be set to be able to compute
$__rate_interval. The default is 15s which does not match the kolla
default of 60s. The symptom of not setting this is that you will see
"no data" when zooming graphs that use rate queries. This occurs as the
interval will be set to a period shorter than the scrape interval.
The recommendation is that you use a common scrape interval for all
jobs. See:

- https://grafana.com/blog/2020/09/28/new-in-grafana-7.2-__rate_interval-for-prometheus-rate-queries-that-just-work/
- https://stackoverflow.com/questions/66369969/set-scrape-interval-in-provisioned-prometheus-data-source-in-grafana

Change-Id: I7e5c1e20c7b66b64cbd333f669ef8d8da60daaa8