cloud-init.service: systemd ordering after after NetworkManager

Autoinstall directives can be provided on the grub cmdline to
cloud-init via kernel parameters like the following:
 autoinstall 'ds=nocloud-net;s=http://somedomain/'

In order to support DNS resolution for NoCloud datasource at
datasource discovery time, cloud-init.service needs to be
orderered after NetworkManager.service and
NetworkManager-wait-online.service
which will have brought up applicable NICs.

Since NetworkManager is After=dbus.service, the cloud-init.service
avoids systemd ordering cycles by also dropping
Before=sysinit.target when it adds, After=NetworkManager.service and
After=NetworkManager-wait-online.service

Add this file overlay for /lib/systemd/system/cloud-init.service
because systemd drop-in files can only add constraints and not
drop prexisting service constraints.

Also add an AUTOMATION_HEADER comment to any generated files to
add discoverability in the event of future bugs/concerns.

LP: #2008952

live-build/functions

@@ -4,6 +4,8 @@
 imagesize=${IMAGE_SIZE:-2361393152}
 fs_label="${FS_LABEL:-rootfs}"
 
+AUTOMATION_HEADER="# Automatically generated by installer build process"
+
 rootfs_dev_mapper=
 loop_device=
 loop_raw=
@@ -1010,6 +1012,7 @@ EOF
         # inform cloud-init of the same (LP: #1982855)
         mkdir -p chroot/etc/cloud/cloud.cfg.d
         cat <<EOF > chroot/etc/cloud/cloud.cfg.d/99-installer-use-networkmanager.cfg
+${AUTOMATION_HEADER}
 # Let NetworkManager manage all devices on this system
 system_info:
   network:
@@ -1017,6 +1020,50 @@ system_info:
     activators: ['network-manager']
 EOF
 
+        # Position cloud-init.service After=NetworkManager.service.
+        # (LP: #2008952).
+        # We override the entire cloud-init.service definition because
+        # systemd drop-in files only allow adding Before/After constraints
+        # yet we are dropping the Before=sysinit.target from the upstream
+        # cloud-init.service.
+        # This override can be dropped when NetworkManager.service can run
+        # Before=sysinit.target when it drops strict dbus.service dependency.
+        cat <<EOF > chroot/lib/systemd/system/cloud-init.service
+${AUTOMATION_HEADER}
+# Based on cloud-init 23.1 for Desktop LiveCD
+[Unit]
+Description=Initial cloud-init job (metadata service crawler)
+DefaultDependencies=no
+Wants=cloud-init-local.service
+Wants=sshd-keygen.service
+Wants=sshd.service
+After=cloud-init-local.service
+After=systemd-networkd-wait-online.service
+# Installer Added After=NetworkManager* ordering
+After=NetworkManager.service
+After=NetworkManager-wait-online.service
+After=networking.service
+Before=network-online.target
+Before=sshd-keygen.service
+Before=sshd.service
+# Before=sysinit.target  Installer removed to allow for NM dependency
+Before=shutdown.target
+Conflicts=shutdown.target
+Before=systemd-user-sessions.service
+
+[Service]
+Type=oneshot
+ExecStart=/usr/bin/cloud-init init
+RemainAfterExit=yes
+TimeoutSec=0
+
+# Output needs to appear in instance console output
+StandardOutput=journal+console
+
+[Install]
+WantedBy=cloud-init.target
+EOF
+
         # Allow cloud-init clean to inform of strict network-manager config
         mkdir -p chroot/etc/cloud/clean.d
         cat <<EOF > chroot/etc/cloud/clean.d/99-installer-use-networkmanager