networkd: do not generate a mac address for vlan interfaces (#3221)

networkd: do not generate a mac address for vlan interfaces (#3221)

While creating a VLAN the mac address should be copied from the parent interface, so that
the VLANs inherit the MAC address of the physical interface.

Before:
```
3: wlp3s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
    link/ether 00:26:c6:85:a3:c2 brd ff:ff:ff:ff:ff:ff
    ...
6: vlan1@wlp3s0: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN group default qlen 1000
    link/ether 22:07:73:9d:43:59 brd ff:ff:ff:ff:ff:ff
7: vlan2@wlp3s0: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN group default qlen 1000
    link/ether 46:30:76:33:35:d4 brd ff:ff:ff:ff:ff:ff
```

After:
```
3: wlp3s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
    link/ether 00:26:c6:85:a3:c2 brd ff:ff:ff:ff:ff:ff
    ...
11: vlan1@wlp3s0: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN group default qlen 1000
    link/ether 00:26:c6:85:a3:c2 brd ff:ff:ff:ff:ff:ff
12: vlan2@wlp3s0: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN group default qlen 1000
    link/ether 00:26:c6:85:a3:c2 brd ff:ff:ff:ff:ff:ff
```

v2 of #1573, with fixed commit message.