allow to discover new devices from dashboard

Instead of having to ssh to the gateway and edit /mnt/permanent/mars_db.yaml to add a new machine, we could launch a new interface from the dashboard, for example a ncurses script, that shows the status of the PDU ports and allows to run a discovery process in the ports that are OFF. This would mean also updating /mnt/permanent/mars_db.yaml from this new script.

A possible output would be something like this:

> PDU 1
	Port 1: Machine1
	Port 2: Machine2
	Port 3: OFF
	Port 4: DISCOVER
	....
	
> PDU 2
   Port 1: DISCOVER
   Port 2: DISCOVER
   Port 3: DISCOVER

@mupuf Please, tell me if this doesn't match what you had in mind.