AutoUpdate is disabled in a manner that can cause it to fail to be disabled in practice

For example:

At some point walinuxagent started doing "auto updating", which we disabled in the distribution (see bug 1650522). We disabled by patching the conffile to explicitly set AutoUpdate=n.

However, if a user had modified that conffile and then automatically updates (for example Xenial has unattended-upgrades enabled by default), then either the user won't see any conffile prompt, or could see one, won't realise the significance of it and choose the default, which is to keep the old configuration.

Then the code will default to AutoUpdate=y because it sees no explicit AutoUpdate=n in the conffile. User behaviour will change from not auto-updating to auto-updating as a result of the SRU.

There are two problems here:

1) SRUs keep changing conffiles. The moment a user changes a conffile, that will stop happening for automatic updates. Then there's no point in changing conffiles for that set of users. If that's OK, then was there any need to change the conffile in the first place?

2) The patch for disabling AutoUpdate should change the default as interpreted by the code when not specified in the conffile, rather than trying to patch the conffile.

Let's track problem 2 in this bug. I filed bug 1657523 to track problem 1.