add dc health panel

add dc health panel

this adds a dc health panel which will be used in the cluster dashboard

it shows the overall cluster status (or simply ok if no cluster defined)
with a symbol and text

then it shows the number of online and offline nodes

optionally it shows then the overall ceph health
of the connected node

if the api call for ceph fails 3 times, we stop it
the ceph status is hidden by default

for updating the cluster and node status, we have to call
updateStatus as a callback from a store which
loads the /api2/json/cluster/status call

we do not have our own store for this, because we will be using the store
which handles this in the component above

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>