any netplan config for wifi devices should not be world readable

Currently, as near as I can tell, curtin writes netplan config to a world readable file in /etc/cloud/ and cloud-init writes it to a world readable file in /etc/netplan. But if there are any wpa2 psks in the config they should be put in a 0600 file.

This doesn't really make any sense for actual clouds, but subiquity should be able to get this right.

One way to do this would be for cloud-init to check through the provided config and put wifis in a separate file or another would be for there to be a way to direct cloud-init to write different parts of the netplan config to different files and a way to set the modes of those files (neither of which appears to be possible today), and for curtin to make use of that. I don't really care :)