netplan / networkd backend / label option not always applied on `netplan apply`

[Nementon]

Hi,

Regarding this feature: https://github.com/CanonicalLtd/netplan/pull/89
The networkd "label" option is not applied on configuration apply "netplan apply" if an ip address/label is already live.

Context:
  - OS: Ubuntu 20.04 (focal)
  - netplan.io: 0.100-0ubuntu4~20.04.3

Considering this `eth1.yml` configuration file:

```
network:
    version: 2
    renderer: networkd
    ethernets:
        eth1:
            addresses:
                - 15.25.25.22/24:
                    lifetime: forever
                    label: 'eth1:1'
                - 15.25.25.26/24:
                     label: default
```

```
$ netplan apply
$ ip a show eth1
eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq master pta-vSwitch state UP group default qlen 1000
    link/ether 00:0c:29:28:c5:cd brd ff:ff:ff:ff:ff:ff
    inet 15.25.25.22/24 brd 15.25.25.255 scope global eth1:1
       valid_lft forever preferred_lft forever
    inet 15.25.25.26/24 brd 15.25.25.255 scope global secondary default
       valid_lft forever preferred_lft forever
```

Then, by modifying the `eth1.yml` configuration file:

```
network:
    version: 2
    renderer: networkd
    ethernets:
        eth1:
            addresses:
                - 15.25.25.22/24:
                    lifetime: forever
                    label: 'anotherlabel'
                - 15.25.25.26/24:
                     label: whoami
```

```
$ netplan apply
$ ip a show eth1
eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq master pta-vSwitch state UP group default qlen 1000
    link/ether 00:0c:29:28:c5:cd brd ff:ff:ff:ff:ff:ff
    inet 15.25.25.22/24 brd 15.25.25.255 scope global eth1:1
       valid_lft forever preferred_lft forever
    inet 15.25.25.26/24 brd 15.25.25.255 scope global secondary default
       valid_lft forever preferred_lft forever
$ ip a flush eth1
$ netplan apply
$ ip a show eth1
3: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq master pta-vSwitch state UP group default qlen 1000
    link/ether 00:0c:29:28:c5:cd brd ff:ff:ff:ff:ff:ff
    inet 15.25.25.22/24 brd 15.25.25.255 scope global anotherlabel
       valid_lft forever preferred_lft forever
    inet 15.25.25.26/24 brd 15.25.25.255 scope global secondary whoami
       valid_lft forever preferred_lft forever
```

I do believe it's not the expected behaviour, current label in yaml configuration file shall be applied during network reconfiguration with `netplan apply`.

If it is the expected behaviour, the documentation should be updated to explain this case.

Best regards,