Add optional prefix parameter to the template

[afbjorklund]

Description

Currently the prefix is hardcoded to /usr/local, for installing guestagent and containerd.

But that doesn't work, if /usr is read-only (like on Container Linux, which uses /opt).

So add an optional parameter.

# Prefix to use for installing guest agent, and containerd with dependencies (if configured)
# 🟢 Builtin default: /usr/local
prefix: null

[afbjorklund]

It worked OK anyway on Fedora CoreOS, which does a symlink to /var:

/usr/local -> ../var/usrlocal

But not on the original CoreOS (Flatcar Container Linux), which does not:

install: cannot create regular file '/usr/local/bin/lima-guestagent': Read-only file system

[afbjorklund]

• Add example for Fedora CoreOS, running ignition not cloud-init #1653

Also needs another Flatcar fix, to set up the symlink from /mnt to /var/mnt.