google-compute-image module: use google services

This adds a few google-specific services to setup the machine.

Accounts are now dynamically created using the google-accounts-daemon,
which allows to click on the "SSH" button in the console and have it
working.

The NixOS image now supports the userdata startup and shutdown scripts.

Misc:
* add all the google services from https://github.com/GoogleCloudPlatform/compute-image-packages/tree/master/google_compute_engine_init/systemd
* add udev rules for disk labels
* synched sysctl rules with https://github.com/GoogleCloudPlatform/compute-image-packages/blob/master/google_config/sysctl/11-gce-network-security.conf