Documentation: Add boot stage of execution to each module documentation

[Brian Vanderbusch]

As a newcomer to cloud-init, the module documentation has been extremely helpful in laying out the options I can utilize. However, I often have to ask myself if i need to do something as a runcmd script rather than utilize a module, since the documentation doesn't clarify what boot stage (or stages as I've come to find out) a particular module is executed in.

Proposal:

Right after the summary of each module in the documentation, add a "Boot stages:" section.

I think this will also help reinforce the concept of the 5 boot stages of cloud-init to newcomers, and that will indirectly make it easier to entice new contributors.

[Dan Watkins (oddbloke)]

So this is a little more complicated than it immediately appears, because the order of modules is configurable at image build time (in /etc/cloud/cloud.cfg).

That said, I think that a disclaimer to this effect at the top of the modules page is probably sufficient to allow us to include the phase that the upstream config (and therefore Ubuntu's, at least).

(We probably also want to have this automated somehow, so that we don't end up with stale documentation if things change phase etc.)

[Brian Vanderbusch (longlivechief)]

That sounds great to me. Of course, that also would lead into documentation on controlling order and phase of modules in /etc/cloud/cloud.cfg.d. Example: https://paste.ubuntu.com/p/77pqCKjrBB/

[James Falcon (falcojr)]

Tracked in Github Issues as https://github.com/canonical/cloud-init/issues/3669