TT#60115 Install only essential packages during Debian bootstrapping

mmdebstrap and debootstrap install any "important" packages in the
standard installation variants, pulling in e.g. libgdbm5 (which was of
priority `important` until #890832 was fixed and it was downgraded to
priority `optional` with libgdbm6).

Same for libusb-1.0-0 vs libusb-0.1-4:amd64:

  % apt-cache show libusb-1.0-0 | grep Priority
  Priority: optional
  % apt-cache show libusb-0.1-4:amd64 | grep Priority
  Priority: important

Resulting in having libusb-0.1-4 *and* libusb-1.0-0 installed on
our NGCP systems, while libusb-0.1-4 is completely unneeded and irrelevant.

So this is unnecessary, and all the other packages that are automatically
pulled in via the standard installation should be handled via according
and explicit dependencies elsewhere, JFTR - currently being in
Debian/buster:

| apt-utils bsdmainutils cpio cron debconf-i18n dmidecode dmsetup
| e2fsprogs gdbm-l10n ifupdown init iproute2 iptables iputils-ping
| isc-dhcp-client isc-dhcp-common kmod less libapparmor1 libapt-inst2.0
| libargon2-1 libbsd0 libcap2 libcap2-bin libcom-err2 libcryptsetup12
| libdevmapper1.02.1 libdns-export1104 libelf1 libestr0 libext2fs2
| libfastjson4  libidn11 libip4tc0 libip6tc0 libiptc0 libisc-export1100
| libjson-c3 libkmod2 liblocale-gettext-perl liblognorm5 libmnl0
| libncurses6 libnetfilter-conntrack3 libnewt0.52 libnfnetlink0 libnftnl11
| libpopt0 libprocps7 libslang2 libss2 libtext-charwidth-perl
| libtext-iconv-perl libtext-wrapi18n-perl libxtables12 logrotate lsb-base
| mount nano netbase procps readline-common rsyslog sensible-utils systemd
| systemd-sysv tasksel tasksel-data tzdata udev vim-common vim-tiny
| whiptail xxd

The essential variant lacks apt(-get), while the minbase variant provides
the same base as essential, *plus* apt so we can actually install
further packages, so that's what we're using.

We pull in systemd + systemd-sysv + init explicitly to force its usage,
ngcp-installer is complaining and prompting otherwise.

We pull in isc-dhcp-client + ifupdown to have a working networking/DHCP
setup after reboot, since we don't use systemd-networkd or alike (yet).

Applying this change will result in lacking the following packages in
our Debian/stretch based trunk builds:

* apt-utils
* blends-tasks
* debconf-i18n
* iputils-ping
* isc-dhcp-common
* libapt-inst2.0
* liblocale-gettext-perl
* libtext-charwidth-perl
* libtext-iconv-perl
* libtext-wrapi18n-perl
* libusb-0.1-4
* logrotate
* tasksel
* tasksel-data
* vim-tiny

The ones we want to have available will be handled via explicit
dependencies in NGCP (meta)packages.

Change-Id: I14dac92d99172cf792a0334601a930ce6698dc83

templates/scripts/includes/deployment.sh

@@ -1422,6 +1422,9 @@ else
   esac
 fi
 
+# install only "Essential:yes" packages plus apt (explicitly included in minbase variant),
+# systemd + network related packages
+DEBOPT_OPTIONS+=("--variant=minbase --include=systemd,systemd-sysv,init,isc-dhcp-client,ifupdown")
 
 if [[ -n "${EFI_PARTITION}" ]] ; then
   if efi_support ; then