debian: install DHCP client and ifconfig packages
debian: install DHCP client and ifconfig packages

When building an image having the elements debian and vm, cloud-init
fails with:

    ci-info: !!!!!!!!Net device info failed!!!!!!!!

Looking at the source code, it executes ifconfig to gather informations
but the `debian` elements does not provide it.  There is also no DHCP
client available which is rather painful.

Install isc-dhcp-client to provide a DHCP client.
Install net-tools to provide ifconfig, required by cloud-init.

Ref: https://phabricator.wikimedia.org/T105152
Change-Id: I76dfd4f87a5c9f08e7c572fb4f5ebeeb34f5f66a

switch-ch/diskimage-builder,switch-ch/diskimage-builder