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>
(cherry picked from commit 52aec32a7da07ca6e9a22ecedde78dafb4b74dfc)
Revert "Install.py: Prevent prompts from breaking apt"

This reverts commit 5995ae7e78dd19f4036f891db9db9fec97d6eab5.

With the changes to ceph-qa-chef and the teuthology kernel task,
we're no longer touching packaged file /etc/grub.d/10_linux, which
was the reason for this apt forcing.  Remove so that we find other
package problems that might be masked by this; we can always
put it back if there are such problems until we can fix those as well.

Signed-off-by: Dan Mick <dan.mick@inktank.com>
(cherry picked from commit c2b0828b19a426c6d73cb2f614333200ae66bac3)
Revert "Revert "Install.py: Prevent prompts from breaking apt""

This reverts commit 67a616a97927efdc4fbcc5edb0d0cf4a724d90e2.

Sigh.  As it turns out, /etc/default/grub being hacked also
causes the same problem.  I think there's a way to fix that cleanly
as well, but until then, replacing the "accept installed version"
hack here so jobs can run.