Create instance-template from instance with multiple disks

#### Description

Currently GCP does not support having a GCP Image with multiple disk associated with it. However, `GCP Instance Templates` allow the attachment of multiple disks. Maybe there is a way to do it in `post-processor` and calling `gcloud` to do it(haven't checked).

#### Use Case(s)

I would like to create a SQL Server Image with part of the configuration data under a different disk and keep it as part of a generic image to be reused in other deployments that reuse the same configuration.

#### Potential configuration

```
instance_template_enabled: true
instance_template_disks: {}
```

#### Potential References