This configures automated installation using the subiquity installer, as
documented at
https://ubuntu.com/server/docs/install/autoinstall-quickstart and other
locations on the Internet.
Installation uses the NFS export of /install, and configures the
installer to use the nocloud-net datasource, creating a per-node
directory containing the processed template file as the user-data file,
and an empty meta-data file.
Kernel and initrd files are pulled from the casper/ directory under the
package dir.
Unnecessary elements of the kernel command line have been removed in the
subiquity installer path.
Support has been added for selecting a subiquity specific default
template, as well as an updated pre-install script and some minor POSIX
shell compliance fixes for the getinstdisk script.
nvme devices were not autosensed as install candidates.
Also, devices smaller than 128 MB are filtered out, as they are likely
to be some media or similar.
