Have a way to accept external data about machines

Maybe a command in the config that is ran on interval to update the machine description (could display information from metalctl). Or have a flag `--machine-cmd` that accepts a command which is run on interval to hotreload the complete config; this would also allow for BMC credential updates.

Start by locating the configuration handling and CLI argument entry points for the proposed interval command or --machine-cmd flag. Compare the two approaches and clarify how external machine descriptions, hot reloads, and BMC credential updates should behave. Done should include an agreed design and tests for the selected behavior.

Written by the indexing model from the issue text.