assistant: error out on set network config for dhcp

assistant: error out on set network config for dhcp

potentially will not happen too often in practice if the sample files
always contain the right source. Still having settings in an answer
file that get ignored does not seem right.

tested with `validate-answer` on a file without `source` in the
network section (which initially caused confusion for me)

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>