compute: calculate or default the `primary_disk_size` task config parameter

The `primary_disk_size` parameter is currently a required config option, meaning it must be set by job authors. This adds additional overhead on job authoring and might cause confusion of what this value needs to be.

It might be advantageous to add a default value when this parameter is not supplied, such as 8Gb. Alternatively, a value could be calculated based on the passed image size.

Start by locating the task configuration handling for `primary_disk_size` and how the required parameter is validated. Compare the proposed fixed default, such as 8Gb, with calculating a value from the passed image size. Done means the project has a decided behavior for omitted values and that behavior is covered by the relevant validation or configuration tests.

Written by the indexing model from the issue text.