Change dashboard order attribute to priority

Dashboards will be ordered in descending order of priority. The word
'priority' seems to signify descending order while 'order' signifies
ascending order.

config/prometheus/common_metrics.yml

 dashboard: 'Environment metrics'
-order: 1
+priority: 1
 panel_groups:
 # NGINX Ingress metrics for pre-0.16.0 versions
 - group: Response metrics (NGINX Ingress VTS)