Migrate availability type to use descriptor

[timja]

Refactoring to cleanup code in preparation for VMSS support.

For users that have an availability set configured and use JCasC they will need to make a minor change to their configuration:

Before:

availabilityType:
  availabilitySet: "vm-agents"

After:

  availabilityType:
    availabilitySet:
      name: "vm-agents"

I've removed the fluent builders around this area as they are a pain to maintain and JCasC is a lot better, its unlikely they are used.

Testing done

Ensured that VMs are still spawning properly

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