Comment 7 for bug 1425376

Sorry, that part of the systemd.special(7) manpage doesn't actually explain the problem with network.target. The problem is described here:

SPECIAL PASSIVE SYSTEM UNITS
       A number of special system targets are defined that can be used to
       properly order boot-up of optional services. These targets are
       generally not part of the initial boot transaction, unless they are
       explicitly pulled in by one of the implementing services. Note
       specifically that these passive target units are generally not pulled
       in by the consumer of a service, but by the provider of the service.
       This means: a consuming service should order itself after these targets
       (as appropriate), but not pull it in. A providing service should order
       itself before these targets (as appropriate) and pull it in (via a
       Wants= type dependency).

network.target is one of these "special passive system units".

Locally fixing these bugs in cloud-init and ifupdown does not unblock the networkless boot.