When coreos-installer is running from the live image, it no longer needs
a separate install image, since by default it installs from content
embedded in the live system.
* Update Fedora CoreOS live PXE and disk install examples to
Fedora 33
* Increase libvirt VM memory from 2GB to 3GB to support live
PXE example, which is mostly just for laptop examples/demos.
Reduce the VM count from 3 to 2 to compensate.
* Change `fedora-coreos.ign` to suggest using an ed25519 SSH
key since Fedora CoreOS 33 disables RSA SHA1 (256 is still ok
but most people won't know which they have)
* Refactor examples to boot provision minimal hosts
with Fedora CoreOS or Flatcar Linux
* Remove the etcd3 cluster example or other specific
kinds of hosts
* Update script get-fedora-coreos
* Remove script get-coreos
