Build custom packer image for GCP

I'm trying to follow the instructions in https://cloudprovider.dask.org/en/latest/packer.html#ec2cluster-with-cloud-init for GCP, but I'm stuck on how to get the packer configuration to recognize the cloud init configuration. For amazon, that works by doing this:

```
{
"builders": [
{
...
"type": "amazon-ebs",
"user_data_file": "/path/to/cloud-init-config.yaml"
}
],
"provisioners": ...
}
```

The closest equivalent (according to https://cloud.google.com/container-optimized-os/docs/how-to/create-configure-instance) for GCP appears to be:

```
{
"builders": [
{
...
"type": "googlecompute",
"metadata": {
"user-data": "/path/to/cloud-init-config.yaml"
}
}
],
"provisioners": ...
}
```

This isn't working for me though. The cloud-init-config.yaml file is being ignored and I'm not sure why.

Do you guys know how to make this work? Have you tried this yet @quasiben?