Comment 20 for bug 1735950

I spoke in IRC to Andres in response to his comment #19. curtin is
correct in telling cloud-init in the installed system to not re-render
/etc/apt/sources.list. However, it would be better if curtin did so using
the newer format where possible.

So, we will update curtin to use the following when cloud-init in the target allows:

apt:
  preserve_sources_list: true

Also, I have written a 'hackmd' doc at
https://hackmd.io/Gx5mVd_nRIixmAOkA2XMAA
that provides information on hte preserve_sources_list setting that
I will also attach here for posterity.

Please read it as I believe it has a complete discussion of this
setting wrt maas's use.