Executor/MaRS/PDU / Dashboard: Add a way to mark a port as being reserved

In our deployments, we plugged the gateways on the same PDU they control to turn on the test machines. This may not be the brightest idea, but having a separate PDU just for the gateway would also be quite wasteful.

Anyway, it would be great if we could document in mars_db.yml that the gateway's port is reserved and should never be turned off. We could then enforce it in the pdu library.