[Doc] Improvement to "Switching Release Channels"

[rosskukulinski]

From this

I think it's important to put a disclaimer that your cloud_config will (probably?) override any changes you make to release channel OR reboot strategy the next time the system boots.

I'm happy to open PR to add an appropriately worded message, but I'd like someone more knowledgeable to sanity check my above assertion.

[robszumski]

@crawford Can you verify that these situations are correct and then we can figure out how best to document:

Manual Change

Machine is booted on stable channel. After boot, /etc/coreos/update.conf is created and specifies beta.
Now: machine tracks stable
After update-engine restart: machine tracks beta
Next Reboot: machine tracks beta

Cloud-config with manual change

Machine is booted on stable channel with cloud-config specifying alpha. After boot, /etc/coreos/update.conf is created and specifies beta.
Now: machine tracks alpha
After update-engine restart: machine tracks beta
Next Reboot: machine tracks alpha

if true, this is misleading/confusing

Cloud-config only

Machine is booted on stable channel with cloud-config specifying beta.
Now: machine tracks beta
Next Reboot: machine tracks beta

[rosskukulinski]

thanks @robszumski - that tracks how I have found update.conf to behave.

[crawford]

This is correct. In fact, we go out of our way to only overwrite the fields in /etc/coreos/update.conf that differ from the cloud-config. This feels incorrect. I don't think we should touch that file.

[robszumski]

@crawford It sounds like this would be better if it matched the drop-in workflow we have for other services. I know thats probably a bigger change to update-engine than we want to make right now.

[crawford]

Yes and yes. For starters, we should fix cloudinit.