dashboard: include smokeping graphs for a device into the dashboard

[aaronkaplan]

Please add the img src=... for smokping images for Graz as well as Vienna into the dashboard.
Ideally, there is a smokeping config, which offers unique file names for the images. Please take a look how this could be done and implement it. If we need to change our smokeping setup, so be it.

[equinox0815]

It would be nice to have a zone wide setting for this and an optional per node/device setting which when present overrides the zone default.
This allows a node admin to use his/hers own smokeping installation for some nodes.

[tanzer]

Please add the img src=... for smokping images for Graz as well as Vienna into the dashboard.

Graz and Vienna need to document how the value for src is determined for an arbitrary device.