The unattended-upgrades package removes older kernel packages before
starting the upgrade process, so that the current kernel and the newly
installed kernel will be retained and all older kernel packages will
be removed.

The removal of older kernel packages requires "apt autoremove" to list
them. So, a setting to enable autoremoval of packages has been turned
on as well.

When tested on a vagrant box, the current kernel 4.17.0.1 and the new
kernel 4.17.0.3 have been retained. Older kernels were removed
preemptively before the installation of 4.17.0.3

Fixes #1372Signed-off-by: Joseph Nuthalapati <njoseph@thoughtworks.com>
Reviewed-by: James Valleroy <jvalleroy@mailbox.org>