dhcp: support FQDN flags in the nettools backend

Make the nettools backed honor the ipv4.dhcp-fqdn-flags property.
14 jobs for bg/dhcp-fqdn-flags in 68 minutes and 23 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #406685
allowed to fail
checkpatch

00:01:08

manual #406690
allowed to fail manual
t_centos:7.5.1804
passed #406691
t_centos:7.6.1810

00:40:15

passed #406694
t_debian:9

00:27:49

passed #406696
t_debian:sid

00:28:04

manual #406695
allowed to fail manual
t_debian:testing
passed #406686
t_fedora:28

00:28:10

passed #406687
t_fedora:29

00:38:47

passed #406688
t_fedora:30

00:34:25

manual #406689
allowed to fail manual
t_fedora:rawhide
passed #406692
t_ubuntu:16.04

00:35:22

passed #406693
t_ubuntu:18.04

00:34:51

 
  External
failed NetworkManager test verification

failed NetworkManager test verification

00:03:04