mgr/dashboard: 'dashboard grafana dashboards update' fails with existing dashboards
When creating a dashboard in Grafana from API (https://grafana.com/docs/http_api/dashboard/#create-update-dashboard), every dashboard JSON 'id' field should be 'null', and the Grafana returns the actual 'id' assigned to that dashboard (self incrementing key value). For updating them, the 'id' needs to match an existing dashboard. If some 'id' is specified and does not matches an existing one, a not found will be triggered. All our dashboard include non-null 'id' fields, so the easy fix would be to remove then (dashboard JSONs also have a 'uid').