Hi Philipp
The standard cloud-init module doesn't work (at least it did not when I tried to use it last year to build an image for SWITCHengines). The cloud-init package itself requires a patch and the version of growpart used by the original module doesn't work either. My customized module also includes a systemd service that waits for the network to be available before cloud-init is started.
The image uses the standard NixOS channel (which is necessary to be able to use nix-channel --update), so the custom stuff needs to be available in the Nix store and pulled in via /etc/configuration.nix (i.e /nix/store/q9n6x4rwskvy86fs3cxyn2dh0gbscjjl-cloud-init-module and /nix/store/qd8ylyvc8gaahivih6zdqslpdxp7y6vf-cloud-init-patch).
You can find the Nix expression that was used to generate the cloud image here: https://github.com/alexandergall/nixos-cloud-image. In particular, the derivation of the custom cloud-init-module is created by simply copying the module to the store: https://github.com/alexandergall/nixos-cloud-image/blob/master/default.nix#L46.
Hope this helps 
--
Alex
