dashboard json cannot contain fixed id when importing from
disk. We used to override this but it didnt caught all problems
so now we block dashboards from beeing imported instead.
closes#10504
Attempting to provision grafana using a previously exported
JSON dashboard with the ID field already set, might result in an error.
In fact, Grafana might believe that a dashboard with that
given ID already exists in the database, throwing an error if it doesn't.
(see pkg/services/sqlstore/dashboard.go#L32)
The dashboard provisioner should set the dashboard id to zero in order to
avoid this behaviour inducing the dashboard to be written to the DB
without ID validation.
