tests/acceptance/boot_linux: rename misleading cloudinit method

tests/acceptance/boot_linux: rename misleading cloudinit method

There's no downloading happening on that method, so let's call it
"prepare" instead.  While at it, and because of it, the current
"prepare_boot" and "prepare_cloudinit" are also renamed.

The reasoning here is that "prepare_" methods will just work on the
images, while "set_up_" will make them effective to the VM that will
be launched.  Inspiration comes from the "virtiofs_submounts.py"
tests, which this expects to converge more into.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Beraldo Leal <bleal@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20210203172357.1422425-3-crosa@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>