MTU not applied to a bridge

[Marian Gasparovic]

Ubuntu 18.04

in a setup like this

```
...
        ens6:
            dhcp4: no
            mtu: 9000
            match:
                macaddress: '18:02:09:14:43:42'
            set-name: ens6
        ens7:
            dhcp4: no
            mtu: 9000
            match:
                macaddress: '18:02:09:14:43:52'
            set-name: ens7
    bonds:
        bondA:
            dhcp4: no
            mtu: 9000
            interfaces:
                 - ens6
                 - ens7
    bridges:
        br-bondA:
            dhcp4: no
            interfaces:
                 - bondA
```

After netplan apply br-bondA keeps MTU 1500, bondA and physical interfaces are at 9000.

Looks like netplan apply does not change bridge MTU at all. I could set it with ip link to whatever value, then netplan apply and mtu is not changed.
It looks like only ip link or system restart (when bridge is recreated) allow to change MTU on this bridge

[Marian Gasparovic (marosg)]

steps to reproduce

5: ens6: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN group default qlen 1000
6: ens7: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN group default qlen 1000

network:
    version: 2
    ethernets:
        ...
        ens6:
            dhcp4: false
            match:
                macaddress: '18:02:18:34:33:49'
            mtu: 1500
            set-name: ens6
        ens7:
            dhcp4: false
            match:
                macaddress: '18:02:18:34:33:59'
            mtu: 1500
            set-name: ens7

adding bondA

        ens6:
            dhcp4: false
            match:
                macaddress: '18:02:18:34:33:49'
            mtu: 1500
            set-name: ens6
        ens7:
            dhcp4: false
            match:
                macaddress: '18:02:18:34:33:59'
            mtu: 1500
            set-name: ens7
    bonds:
        bondA:
            dhcp4: false
            mtu: 2000
            interfaces:
                    - ens6
                    - ens7

netplan apply
5: ens6: <BROADCAST,MULTICAST,SLAVE,UP,LOWER_UP> mtu 2000 qdisc fq_codel master bondA state UP group default qlen 1000
6: ens7: <BROADCAST,MULTICAST,SLAVE,UP,LOWER_UP> mtu 2000 qdisc fq_codel master bondA state UP group default qlen 1000
7: bondA: <BROADCAST,MULTICAST,MASTER,UP,LOWER_UP> mtu 2000 qdisc noqueue state UP group default qlen 1000

adding br-bondA

        ens6:
            dhcp4: false
            match:
                macaddress: '18:02:18:34:33:49'
            mtu: 1500
            set-name: ens6
        ens7:
            dhcp4: false
            match:
                macaddress: '18:02:18:34:33:59'
            mtu: 1500
            set-name: ens7
    bonds:
        bondA:
            dhcp4: false
            mtu: 2000
            interfaces:
                    - ens6
                    - ens7
    bridges:
        br-bondA:
           dhcp4: false
           mtu: 3000
           interfaces:
                   - bondA

netplan apply
5: ens6: <BROADCAST,MULTICAST,SLAVE,UP,LOWER_UP> mtu 2000 qdisc fq_codel master bondA state UP group default qlen 1000
6: ens7: <BROADCAST,MULTICAST,SLAVE,UP,LOWER_UP> mtu 2000 qdisc fq_codel master bondA state UP group default qlen 1000
7: bondA: <BROADCAST,MULTICAST,MASTER,UP,LOWER_UP> mtu 2000 qdisc noqueue master br-bondA state UP group default qlen 1000
8: br-bondA: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 2000 qdisc noqueue state UP group default qlen 1000 <= MTU not set to 3000

later another tests

ip l set br-bondA mtu 8888
5: ens6: <BROADCAST,MULTICAST,SLAVE,UP,LOWER_UP> mtu 9000 qdisc fq_codel master bondA state UP group default qlen 1000
6: ens7: <BROADCAST,MULTICAST,SLAVE,UP,LOWER_UP> mtu 9000 qdisc fq_codel master bondA state UP group default qlen 1000
7: br-bondA: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 8888 qdisc noqueue state UP group default qlen 1000 <= set correctly
8: bondA: <BROADCAST,MULTICAST,MASTER,UP,LOWER_UP> mtu 9000 qdisc noqueue master br-bondA state UP group default qlen 1000

ip l set br-bondA mtu 8887
5: ens6: <BROADCAST,MULTICAST,SLAVE,UP,LOWER_UP> mtu 9000 qdisc fq_codel master bondA state UP group default qlen 1000
6: en...