Decouple from ubuntu-cdimage & debian-cd landings

We don't want to regress the ubuntu desktop traditional image when
livecd-rootfs lands uncordinated with ubuntu-cdimage & debian-cd.
Decoupling enables us to:
- land livecd-rootfs (no change for ubuntu-desktop project,
ubuntu-desktop:ubiquity-ng can be tested once the 2 others branches
lands)
- land ubuntu-cdimage & debian-cd for publishing any arbitrary squashfs
names
- finally, revert that commit and land it in livecd-rootfs to transform
the traditional ubuntu-desktop image to a layered approach.

live-build/auto/config

@@ -390,19 +390,23 @@ esac
 
 case $PROJECT in
 	ubuntu|ubuntu-dvd)
-		PASSES="install-minimal install live"
-		add_task install-minimal minimal standard ubuntu-desktop-minimal ubuntu-desktop-minimal-default-languages
-		add_task install ubuntu-desktop ubuntu-desktop-default-languages
 		LIVE_TASK='ubuntu-live'
 
 		case ${SUBPROJECT:-} in
 			ubiquity-ng)
+				PASSES="install-minimal install live"
+				add_task install-minimal minimal standard ubuntu-desktop-minimal ubuntu-desktop-minimal-default-languages
+				add_task install ubuntu-desktop ubuntu-desktop-default-languages
+
 				# LANG PASS for minimal and install
 				add_layered_pass_delta install-minimal desktop-minimal-default-languages '^desktop-minimal-(?!default-languages)[^.]+$'
 				add_layered_pass_delta install-minimal desktop-minimal-default-languages ''  # none (if no default langpack is selected)
 				add_layered_pass_delta install desktop-default-languages '^desktop-(?!default-languages|minimal|common)[^.]+$'
 				add_layered_pass_delta install desktop-default-languages ''  # none (if no default langpack is selected)
 				;;
+			*)
+				add_task install minimal standard ubuntu-desktop ubuntu-desktop-minimal-default-languages ubuntu-desktop-default-languages
+				;;
 		esac
 
 		case $ARCH in