dhcp: minor refactoring to switch default IPv4 DHCP plugin to "nettools" with one-line change

Minor refactoring so that there is only a one-line change necessary to
flip the implementation of the "internal" DHCP plugin for IPv4 from
"systemd" to "nettools".

We don't do that yet, because there are still some issues (e.g. the
lease is not persisted for nettools plugin). Eventually we want to
switch, so prepare the code to be almost there.
15 jobs for th/dhcp-factory-cleanup in 49 minutes and 54 seconds (queued for 9 seconds)
Status Job ID Name Coverage
  Test
passed #497294
allowed to fail
checkpatch

00:00:58

manual #497299
allowed to fail manual
t_centos:7.5.1804
passed #497300
t_centos:7.6.1810

00:23:41

passed #497306
t_debian:10

00:19:14

passed #497305
t_debian:9

00:16:17

passed #497308
t_debian:sid

00:19:07

manual #497307
allowed to fail manual
t_debian:testing
passed #497295
t_fedora:28

00:25:38

passed #497296
t_fedora:29

00:49:51

passed #497297
t_fedora:30

00:41:59

manual #497298
allowed to fail manual
t_fedora:rawhide
passed #497301
t_ubuntu:16.04

00:19:36

passed #497302
t_ubuntu:18.04

00:19:08

manual #497304
allowed to fail manual
t_ubuntu:devel
manual #497303
allowed to fail manual
t_ubuntu:rolling