Default Gateway option configures gateway in the wrong routing table

When using the set_default_gateway API, the default gatway gets set in routing table 1 instead of the default linux routing table. Heres the section of netplan that gets generated regarding that

```
routes:
- table: 1
    to: 0.0.0.0/0
    via: 10.27.0.1
routing-policy:
- from: 10.27.0.0/16
    priority: 100
    table: 1
- from: 10.27.0.0/16
    table: 254
    to: 10.27.0.0/16
```

In my case, I am setting a vlan interface as the default gateway. However there is a bond interface thats also getting created with a `gateway4` option, and it seems that is being used as the default gateway still after the node is deployed.

I am seeing this in MAAS 3.4rc2.