prevent old packages from being removed via "apt autoremove"

[prakashsurya]

As discussed in this pull request, and more specifically the comment thread here, it may be possible for a Delphix appliance to wind up in a non-functional state after running apt autoremove.

I haven't verified this is possible, but the scenario that I'm worried about is this:

1. Appliance is initially starts with delphix-kernel-4.15.0-45-generic package installed
2. Appliance is upgraded, thus installing the delphix-kernel-4.18.0-45-generic package
3. apt autoremove is run on Appliance
4. package delphix-kernel-4.15.0-45-generic is removed as a result of (3)
5. ubuntu kernel package for kernel version 4.15.0-45-generic remains installed (due to /etc/apt/apt.conf.d configuration supplied by ubuntu's kernel package)
6. Appliance attempts to boot into kernel 4.15.0-45-generic (e.g. via manually selecting it in GRUB)
7. Appliance fails to boot due to ZFS modules corresponding to 4.15.0-45-generic have been removed as a result of (4)

We should first verify if this situation is possible, and then if it is, ensure we prevent it from occurring; e.g. by providing proper configuration in /etc/apt/apt.conf.d to prevent old delphix-kernel packages from being auto-removed, similar to ubuntu's kernel packages.