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.