ethernet: don't set a hw address in a default connection

Use an ifname instead.

This also fixes creation of default wired connections for cheapo USB
ethernet adapters that come with the EEPROM footprint unpopulated.
12 jobs for lr/default-ethernet-ifname in 32 minutes and 40 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Test
passed #285835
allowed to fail
checkpatch

00:01:00

passed #285842
t_debian:9

00:13:43

passed #285844
t_debian:sid

00:15:59

manual #285843
allowed to fail manual
t_debian:testing
passed #285836
t_fedora:28

00:23:47

passed #285837
t_fedora:29

00:30:31

passed #285838
t_fedora:30

00:32:17

manual #285839
allowed to fail manual
t_fedora:rawhide
passed #285840
t_ubuntu:16.04

00:22:16

passed #285841
t_ubuntu:18.04

00:13:57

 
  External
failed NetworkManager test verification

failed NetworkManager test verification

06:37:15