Allow huge extra manifests

Calico Enterprise requires enormous manifests to install - around 2.25MB worth, which in an unscientific test still came to about 550KB when base64-encoded and gzipped. There is an open source version of Calico that is smaller, and it is possible to upgrade to Calico Enterprise from that, but Tigera does not support that in production. Since Calico is used as the SDN for the cluster, it must be installed on Day 1.

The CoreOS ISO limits Ignition space to 256KB, so there is no way to fit manifests of this size.

However, in practice since 4.13 we repack the ISO ourselves in order to add the agent-tui binaries. To date, we have kept a check that the Ignition image does not exceed the size of the placeholder image in the ISO (i.e. 256KB). This is to ensure that the ISO appears like one customised by coreos-installer, and is not corrupted when further coreos-installer commands are used on it. We could allow larger Ignition images if we also rewrote the header that indicates the size of the Igntion image.