Add python3-systemd to our minimized builds

This package is needed by ubuntu-advantage-tools for cloud images
only. u-a-t ultimately will drop this entirely in their next release.
(cf: https://github.com/canonical/ubuntu-pro-client/issues/2692)

So instead of putting this in the seeds and then having to deal
with the seed changes to Mantic (after its release) and SRU of
ubuntu-meta, I'd rather have changes in livecd-rootfs done. Once
python3-systemd dependency is dropped from u-a-t, we'll drop it
from livecd-rootfs in Mantic+, too.

live-build/auto/config

@@ -1102,8 +1102,15 @@ case $PROJECT in
 		    # or by passing --no-install-recommends to apt-get install.
 		    # Apt config is set using `APT_OPTIONS` variable in this script.
 		    # This fixes LP: #2031640
+		    #
+		    # We're also installing python3-systemd for now because it's needed by
+		    # ubuntu-advantage-tools. This will eventually be dropped in the next
+		    # release of u-a-t. This is being discussed and tracked at
+		    # https://github.com/canonical/ubuntu-pro-client/issues/2692.
+		    # Once python3-systemd is dropped from u-a-t, we can remove it
+		    # from here, too.
 		    APT_OPTIONS="${APT_OPTIONS:+$APT_OPTIONS }--no-install-recommends"
-			add_package install ubuntu-cloud-minimal
+			add_package install ubuntu-cloud-minimal python3-systemd
 		else
 			add_task install minimal standard cloud-image
 			add_package install ubuntu-minimal