cloud-config.service: run After snap.seeded.service.

This makes cloud-config.service (and as a result cloud-final.service)
run After snap.seeded.service.  This is required to ensure that
pre-seeded snaps can be used by cloud-init or user-data input.

The snap.seeded.service was added to snapd at:
  canonical/snapd#5124

Note that the following would be a workaround:
 snap:
  commands:
   00: snap wait system seed.loaded

LP: #1767131

systemd/cloud-config.service.tmpl

@@ -2,6 +2,7 @@
 [Unit]
 Description=Apply the settings specified in cloud-config
 After=network-online.target cloud-config.target
+After=snapd.seeded.service
 Wants=network-online.target cloud-config.target
 
 [Service]