Code review comment for lp:~blake-rouse/maas/fix-1646571

I'm on the fence between approving this branch and Needs Information. I think this is generally okay; it should work for standard Ubuntu images and will ensure the apt configuration is consistent.

Bu will this break custom images which use an apt configuration different than what cloud-init pulls down?

Here's where it originally landed:

http://bazaar.launchpad.net/~maas-committers/maas/trunk/revision/1172?start_revid=1172

So the original reasons were:

 - Not breaking custom (ARM) configurations.
 - Not overwriting "a local mirror that was specified somewhere else" (I assume specified in the image, as I suggested earlier, but might this cause a deployment to revert to MAAS repositories on future boots, even after someone deploys a node and customizes it?)

Just going to throw this out there, but if someone deploys a different Debian-based distribution with MAAS, would this change break them, too, by overwriting the Debian repositories with MAAS repositories?

Given that this is already used with the APT v3 core (when was that introduced? assuming we haven't heard any complaints?) then I have to say, this is probably safe. So I'll approve, but let's think about the above before landing this.