Support for specifying VM Image for Cloud Batch backend

[JackAtScaleSec]

Certain organizations utilizing dsub may require the ability to specify a custom VM image for compliance, regulatory, or internal requirements. The underlying Cloud Batch API supports specifying an image when creating a job.

I'm requesting adding support for specifying a custom VM image when creating a dsub job using the Google Cloud Batch backend.

FYI - I requested access to the Contributor License Agreement as referenced in CONTRIBUTING.md but still haven't been granted access to the file.