GCE: make no_*_change() API flexible enough to be used by the instance itself
GCE: Common: introduce no_property_change() API and use it where possible, get rid of "done." messages
GCE: Instance: warn if start() causes the IP address to change
GCE: Instance: factor out the common config into NixOS
GCE: Images: initial implementation
GCE: Instance: requre the user to create a bootstrap image; allow disk image options to also refer to image resources