[Merge] ~mwhudson/livecd-rootfs/+git/livecd-rootfs:layer-fixes into livecd-rootfs:ubuntu/master

[Michael Hudson-Doyle]

Michael Hudson-Doyle has proposed merging ~mwhudson/livecd-rootfs/+git/livecd-rootfs:layer-fixes into livecd-rootfs:ubuntu/master.

Commit message:
   * Fix a few things found while porting the liver server ISO builds to
     layers.


Requested reviews:
  Ubuntu Core Development Team (ubuntu-core-dev)

For more details, see:
https://code.launchpad.net/~mwhudson/livecd-rootfs/+git/livecd-rootfs/+merge/404721

I've been working on porting the live server ISO builds over to the layers stuff (finally!) and these are the changes I ended up making to the generic machinery. It probably makes sense to review each commit individually (and read the commit messages). I'll do a ubuntu:canary build with and without this change to make sure I haven't broken the existing use of layers.
-- 
Your team Ubuntu Core Development Team is requested to review the proposed merge of ~mwhudson/livecd-rootfs/+git/livecd-rootfs:layer-fixes into livecd-rootfs:ubuntu/master.
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