kernel.py: put submenu name in 01_ceph_kernel if necessary

We had been writing 01_ceph_kernel with the kernel title, and
relying on the fact that grub.cfg would never have submenus in it
(implemented by a hack to /etc/grub.d/10_linux which neutered its
submenu creation).  However, that hack was modifying a package file,
and got in the way of later apt commands.  Rather than doing it
that way, this divines the title of the submenu and sets the
default variable to "submenu>kernel", which works to select the
desired kernel.

It depends on there being only one level of submenu, and on the
format of the menuentry and submenu commands, dictated by grub2.
None of this is likely to work at all outside Ubuntu.

Fixes: #4496
Signed-off-by: Dan Mick <dan.mick@inktank.com>
Reviewed-by: Dan Mick <dan.mick@inktank.com>