Task `Check if extra configuration file exists` picks up all files in
`{{ node_custom_config }}/grafana` including those that get handled
specially later on.
While `prometheus.yml` and `provisioning.yml` are best excluded from
extra config , because their treatment requires more than just copying,
`grafana_home_dashboard.json` may simply be treated as extra config,
which saves the execution of two additional tasks.

Closes-Bug: 2067999

Change-Id: I7bce1fe3d0a96816f1782107b202d6dac7d1291d
Signed-off-by: Jan Horstmann <horstmann@osism.tech>