Fix: Instance level stateful disk config

[beardedsamwise]

There are a couple of issues with the compute-mig module with respect to instance level stateful disks:

1 - The example and documentation is misleading and inaccurate. You don't need to know the instance name to create an instance level stateful config, you define the instance you want to create within the MIG and attach any disks you want as stateful
2 - There is a bug in the stateful config where the ID is passed for the instance group manager which results in a 404 when applying.

The requested URL <code>/compute/v1/projects/redactedproject/zones/australia-southeast1-b/instanceGroupManagers/projects/redactedproject/zones/australia-southeast1-b/instanceGroupManagers/mongodb/createInstances?alt=json</code> was not found on this server.

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[ludoo]

Can you fix tests?