autoinstall: set get_domain=unassigned on jessie

d-i's netcfg has a bug where the DHCP domain name option gets ignored
for preseeding purposes and manual input is required. I debugged this
extensively and filled it as https://bugs.debian.org/772523.

As we run the installer with priority=high (the default), meaning that
we get prompted for >= high questions, this breaks automated installs
for Debian. The bug exists in Ubuntu as well, but Ubuntu has a custom
patch on top of netcfg that downgrades that question to priority=medium.
The reason for this change is unknown, as in a typical Ubuntu fashion,
their custom patch is undocumented in the Ubuntu source and was never
forwarded to Debian along with a rationale. It's possible it's a
ill-conceived fix for the bug above.

Besides pushing for a fix in time for jessie, we have the following
options for a workaround:
1) Set d-i to priority=critical. This may be a good idea anyway but as
this touches far more that netcfg, it's not a great workaround right
now.
2) Remove "option domain-name" from our DHCP config; d-i would resort to
the domain found in the FQDN and the bug would not manifest. This is
not a very elegant solution.
3) Trick d-i by setting netcfg/get_domain to "unassigned" (or any other
value) in the kernel command-prompt, similar to how we set
netcfg/get_hostname to "unassigned" as well. This affects only the
bootstrap phase of the installer anyway, as immediately after netcfg
comes preseeding and we preseed get_domain anyway, under our subnets/
configuration.

This implements solution (3). As the bug doesn't manifest in Ubuntu (as
explained above), this currently touches only the jessie config.

Change-Id: Ibd8cb5d4dab0b8480e09178d06d4fc4cccc68577
autoinstall: switch subnet detection to default gw

Our preseeding configuration currently checks the subnet's broadcast
address and performs a switch on that, to include the respective
subnets/*.cfg config.

Unfortunately, this now impossible with the Debian d-i. Ubuntu ships
with both udhcpc and isc-dhcp-client in the netboot initrd and netcfg is
(hard)coded to prefer dhclient over udhcpc, so Ubuntu ends up using
dhclient for the DHCP configuration. In contrast, Debian doesn't ship
dhclient, so it just uses udhcpc for the config. udhcpc's default-script
does not pass brd to "ip address" and thus does not set the broadcast
address on the interface, so our "ip address show" call returns garbage.

Moreover, d-i doesn't ship with ifconfig at all nowadays (neither
trusty's nor jessie's installers ship with it) but rely exclusively on
iproute2 instead, and iproute2 doesn't have an option to show the
broadcast address of an interface at all. The d-i environment is severly
limited (no awk, perl, or anything fancier than that e.g. ipcalc) and
calculating the broadcast address based on the IP/CIDR mask in a
one-liner using busybox's /bin/sh while (probably) possible, is a bit...
extreme.

Another solution would be to bundle the isc-dhcp-client udeb into a
gzipped cpio and append it to jessie's initrd, therefore emulating
Ubuntu. I approached this solution but it got messy real quick (multiple
library udeb dependencies, glibc incompatibilities etc.)

Instead, switch our simple switch/esac to use "ip route list | grep
default" and those subnets' default gateways, which is guranteed to work
across all of our systems.

Change-Id: I1624d7cd5d68c1a31ac49a1a452dcd2adc960ed7