New Desktop Installer

Project on hold until further decision.

Specifications:
Multi layer file system: https://docs.google.com/document/d/1pi3RT8bb2yjmC-mn4Fam_akkYChAwFAlkVcF_G_a6GY

MPs:
https://code.launchpad.net/~jibel/livecd-rootfs/+git/add_multi_layered_squashfses_support/+merge/360878 : MERGED

https://code.launchpad.net/~jibel/debian-cd/support_for_multilayer_images/+merge/359228 : MERGED
https://code.launchpad.net/~jibel/ubuntu-cdimage/support_for_multilayer/+merge/364105 : MERGED

https://code.launchpad.net/~jibel/curtin/+git/add-layer-images-support/+merge/359761 : MERGED

Work items:
Check file content diff chroot.*: DONE
Create squashfses + diff: : DONE
Create seeds corresponding to stages: DONE
debiancd - Support generic squashfs names: DONE
debiancd - Generate multiple squashfses and manifest: DONE
Hack casper: DONE
Seeds to be reviewed by archive admin: DONE
Verify that tasksel lists the ubuntu-desktop-minimal: DONE
Add snaps to images: DONE
Split languages in minimal and defaults sets: DONE
Create negative langpacks - seed part: DONE
Create negative langpacks - livecdrootfs part: DONE
Teach layered image to lb_binary: DONE
Test image builds - desktop layered: DONE
Test image builds - desktop "standard": DONE
Test image builds - server live: DONE
Test image builds - ubuntu-mate: DONE
ubuntu-cdimage - Retrieve several build artifacts of the same type (eg squashfs, manifest, size): DONE
Update update-manager with new tasks: TODO
Register subproject desktop job on launchpad: TODO
Add multiple Task-Keys (one package per language) to ensure Task lang isn't set as installed apart from live: DONE
curtin - Add multilayer support: DONE
curtin - Test entire desktop installation with a yaml config file: DONE