When the installer generates a CPMS, it should only add the `failureDomains` field when there is more than one failure domain. When there is only one failure domain, the fields from the failure domain, eg the zone, should be injected directly into the provider spec and the failure domain should be omitted.

By doing this, we avoid having to care about failure domain injection logic for single zone clusters. Potentially avoiding bugs (such as some we have seen recently).

IIRC we already did this for OpenStack, but AWS, Azure and GCP may not be affected.

Can be demonstrated on Azure on the westus region which has no AZs available. Currently the installer creates the following, which we can omit entirely:
```
failureDomains:
  platform: Azure
  azure:
  - zone: ""
```

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