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.
