This patch adds a default panel that will be used when another service
will add a new panel into the main dashboard through grains. This is the
common part between all panels of the main dashboard.
It replaces the static main dashboard that has been removed.
This patch provides the main dashboard through grains. It is not the final
way to provide the main dashboard. It is more a workaround because we
don't know yet how to compose a dashboards with pieces split between
several formulas. We will probably use YAML pieces.