[bug#74389] [PATCH] services: add cloud-init service

[Alex]

Ricardo,

This patch is just a MVP for implementing a cloud-init service. The goal 
primarily was to get the networking, disk and filesystem mounts, and ssh 
authentication working properly.

>From a high level this patch implements a cloud-init-service which uses 
>cloud-init functionality to gather information from it's run-time environment 
>and then run cloud-init modules in order to properly configure the vm. It 
>provides a configuration interface for choosing which modules to run as well 
>as a mechanism to add extra configuration files.

I took a few major shortcuts in order to get a working image.

The first is that instead of writing guix specific functionality for cloud-init 
modules, I decided to attempt to use the ubuntu (which I believe is the 
default) tools. This is why the cloud-utils package as well as the 
network-manager-service-type are included in the vm image template I created as 
part of this patch.

The second shortcut is that I didn't see a good way to guix-ify the cloud-init 
service definitions/runtime environment (i.e. I used the /etc/cloud directory 
to handle most of the configuration files). I'd prefer if configuration lives 
in the store, but the configuration files and the gathered data live in 
locations which I didn't see a way to overwrite easily. I'm sure this exists, 
but would require more digging into the cloud-init source code and/or patches 
that would need to be pushed upstream.

The final shortcut is the actual running of the cloud-init-service. I didn't 
see a good way to *wait* for a process to complete, i.e. to block other 
services until the current service completes. It looks to me that running a 
long-running one-shot service (each stage of cloud-init typically takes a few 
seconds to run) doesn't block any service which requires it's provisions. Maybe 
there's some shepherd nuances that I'm not aware of. I ended up creating a 
computed file which produces a shell script which runs each cloud-init stage in 
sequence as a workaround to a more robust solution.

I added some extra comments with instructions on how to add extra cloud-init 
configuration files. This is required for cloud-init to run properly on 
DigitalOcean (the order of the data sources needs to be changed from the 
default). It might be necessary for other p

I've tested this on digital ocean and gotten it work properly (and then used 
guix deploy to update the configuration of the image). 

NOTE: there are some comments in the template file that would need to be 
uncommented to build the exact image I used.

Using this patch it should be easy to create an image for testing using:

./pre-inst-env guix system image -t qcow2 
gnu/system/examples/cloud-init-image.tmpl

I would appreciate further testers and any suggestions for improvement.

Thanks!

Alex