Enhance the initial, basic network configuration

[Frank Heimes]

With the support of the subiquity live installer on s390x, one needs to initially specify the basic network configuration.
This can either be done as part of the parm file (means by specifying appropriate kernel arguments) or interactively at the serial console - after the installer kernel and initrd is up and before subiquity is active. This here is about the latter, interactive, option.

An example network configuration may look like this:
"
...
Unable to find a medium container a live file system
Attempt interactive netboot from a URL?
yes no (default yes):
yes
Available qeth devices:
0.0.c000
zdev to activate (comma separated, optional):
c000
QETH device 0.0.c000:0.0.c001:0.0.c002 configured
Two methods available for IP configuration:
* static: for static IP configuration
* dhcp: for automatic IP configuration
static dhcp (default 'dhcp'):
static
ip:
10.11.12.11
gateway (default 10.11.12.1):
10.11.12.1
dns (default 10.11.12.1):
10.11.12.1
vlan id (optional):
1234
http://cdimage.ubuntu.com/ubuntu-server/daily-live/current/groovy-live-server-s390x.iso (default)
url:
ftp://installserver:21/ubuntu-live-server-20.10/groovy-live-server-s390x.iso
http_proxy (optional):

Configuring networking...
IP-Config: encc000.1234 hardware address 3e:4f:1a:2b:2c:3d mtu 1500
IP-Config: encc000.1234 guessed broadcast address 10.11.12.255
IP-Config: encc000.1234 complete:
address: 10.11.12.11 broadcast: 10.11.12.255 netmask: 255.255.255.0

gateway: 10.11.12.1 dns0 : 10.11.12.1 dns1 : 0.0.0.0

rootserver: 0.0.0.0 rootpath:
filename :
Connecting to install-server:21 (10.13.14.15:21)
groovy-live-server-s 10% |*** | 72.9M 0:00:08 ETA
...
"

And administrator that does an installation needs to be very careful not to make any typos and specify wrong data (like accidentally pointing to a former point release 20.04.(n-1) instead of 20.04.n), since there is no way to update an entry. In such a case the installation needs to be restarted from scratch (like on z/VM re-executing the REXX script or on LPAR redoing the "Load from Removable Media or Server" HMC task).

It looks like people specify relatively often a wrong URL at the end (I at least heard about it several times from customers and partners), which leads to the fact that the ISO cannot be found or booted.
Having the option for a final confirmation (before going forward) and to go back (and forth) with the different config options would allow to adjust wrong entries - and being able to redo the entire basic network config in case the ISO could not be found or booted - would be nice and a considerable improvement of user experience at this early install phase.

Furthermore it would be great if:
- at stage "zdev to activate (comma separated, optional)"
  one could select the first qeth device in the list by just hitting Enter
- at stage "static dhcp (default 'dhcp')"
  we could change the default from 'dhcp' to 'static' (which I believe is more common on s390x)
- at stage "ip"
  one could also specify the ip address in CIDR notation (like it was possible in d-i)
  that would be especially helpful in case a non full /24 networks are used.