CFT GCE instance example raises warning

> code: EXTERNAL_API_WARNING
> data:
> - key: disk_size_gb
> value: '100'
> - key: image_size_gb
> value: '10'
> message: "Disk size: '100 GB' is larger than image size: '10 GB'. You might need to\
> \ resize the root repartition manually if the operating system does not support\
> \ automatic resizing. See https://cloud.google.com/compute/docs/disks/persistent-disks#repartitionrootpd\
> \ for details."
>

```
properties:
name: foo-instance
zone: us-central1-a
diskImage: projects/ubuntu-os-cloud/global/images/family/ubuntu-1804-lts
diskSizeGb: 100
machineType: f1-micro
diskType: pd-ssd
network: default
metadata:
items:
- key: startup-script
value: sudo apt-get update
```

cc @ocsig