[Merge] lp:~jibel/livecd-rootfs/seed_snap_on_layered_squashfs into lp:livecd-rootfs

[Jean-Baptiste Lallement]

Jean-Baptiste Lallement has proposed merging lp:~jibel/livecd-rootfs/seed_snap_on_layered_squashfs into lp:livecd-rootfs with lp:~jibel/livecd-rootfs/add_multi_layered_squashfses_support as a prerequisite.

Commit message:
Adds support for seeding snap package on stacked squashfses images.

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

For more details, see:
https://code.launchpad.net/~jibel/livecd-rootfs/seed_snap_on_layered_squashfs/+merge/358840

This patch allows to seed snaps on specific layers of stacked squashfses images. The list of snap to seed on each layer is defined in the image seed.
The final build artefacts include a delta manifest for each layer with the snaps installed on the layer and a full manifest with all the snaps installed when all the squashfses are mounted.

Additionally, there is a small code refactoring to move code to the functions script and prevent reinstalling already install snap package during image build if the same snap is included more than once in a seed. 
-- 
Your team Ubuntu Core Development Team is requested to review the proposed merge of lp:~jibel/livecd-rootfs/seed_snap_on_layered_squashfs into lp:livecd-rootfs.
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