Safe power down

[mmaelicke]

Right now, the cluter is shut down by disconnecting all nodes and the Raspberry Pi 3+ from the voltage supply while running. This can corrupt the OS on the SD card and should be improved. I would like to have a solution with a power down button outside the casing:

1. User pushes a power-down button connected to the Raspi GPIO. As respoonse, the Raspi turns an indicator LED on.
2. The raspi sshs into all nodes and calls a shutdown script (or just issues the shutdown -h now command)
3. (optional) wait until the connection to each of the nodes is closed (meaning the node is powered off)
4. alternatively, wait for 30 secs or 1 minute until all nodes are shutdown
5. shut the raspi down. As soon as it is powered down, the indicator LED shut be off as the GPIOs are not supplied anymore (are they?).

Now, the User can use the hard power switch and disconnect the whole cluster from supply.