EKS-A Nutanix Provider Action Items

These are some of the action items that came up from bugbash. These items are roughly in the order by priority
- [ ] Support auto-generating SSH keys when `sshAuthorizedKeys` field is not set or is empty
- [x] Update CAPX in build-tooling to support `additionalTrustBundle`
- [x] Update EKS-A to pass `additionalTrustBundle` to CAPX
- [x] Propagate UUID to CAPX if provided
- [x] Rename Nutanix credentials env vars from `NUTANIX_USER` to `EKSA_NUTANIX_USERNAME` and `NUTANIX_PASSWORD` to `EKSA_NUTANIX_PASSWORD` to be consistent with other EKS Anywhere providers
- [x] Add preflight to validate connection to `NutanixDatacenterConfig`'s `endpoint:port` using something similar to this https://github.com/aws/eks-anywhere/blob/2a03ef2bb651cfc70c1cfea12a032103263d7817/pkg/executables/govc.go#L598-L604
- [x] Add preflight to validate Nutanix credentials before validating subnets and images
- [x] Add preflight to validate `NutanixDatacenterConfig.endpoint` is a valid endpoint
- [x] Add preflight to validate `NutanixDatacenterConfig.port` is a valid port
- [x] Add preflight to validate `NutanixDatacenterConfig.additionalTrustBundle`, if provided, is a valid PEM encoded cert
- [x] Add preflight to validate `NutanixMachineConfig`'s `memorySize`, `systemDiskSize`, `vcpuSockets` and `vcpusPerSocket`
- [x] Separate out validation errors for cluster/subnet/image instead of wrapping them https://github.com/aws/eks-anywhere/blob/main/pkg/providers/nutanix/validator.go#L45-L55
- [x] In the `NutanixDatacenterConfig` object, include the `additionalTrustBundle` by default in the generated cluster config
- [x] In the `NutanixDatacenterConfig` object, change the default port to something else since 0 is not a valid port. Maybe set it to the default Prism Central port `9440`?
- [x] In the `NutanixMachineConfig` object, don't include `uuid: null` by default in the generated cluster config