Use json.Marshal to correctly handle unmarshalling of vSphere Username

time="2023-09-25T21:29:07Z" level=error msg="error running openshift-install create manifests, stdout: level=warning msg=failed to parse first occurrence of unknown field: failed to unmarshal install-config.yaml: error unmarshaling JSON: while decoding JSON: json: unknown field \"username\"\nlevel=info msg=Attempting to unmarshal while ignoring unknown keys because strict unmarshaling failed\nlevel=error msg=failed to fetch Master Machines: failed to load asset \"Install Config\": failed to create install config: invalid \"install-config.yaml\" file: [platform.vsphere.vcenters.username: Required value: must specify the username, platform.vsphere.failureDomains.server: Invalid value: \"vcenterplaceholder\": server does not exist in vcenters]\n" func="github.com/openshift/assisted-service/internal/ignition.(*installerGenerator).runCreateCommand" file="/src/internal/ignition/ignition.go:1688" cluster_id=aad05e8e-a9fe-4f60-b580-0b2f6c4fdf10 error="exit status 3" go-id=1569 request_id=
time="2023-09-25T21:29:07Z" level=error msg="failed generating install config for cluster aad05e8e-a9fe-4f60-b580-0b2f6c4fdf10" func="github.com/openshift/assisted-service/internal/bminventory.(*bareMetalInventory).generateClusterInstallConfig" file="/src/internal/bminventory/inventory.go:1755" cluster_id=aad05e8e-a9fe-4f60-b580-0b2f6c4fdf10 error="error running openshift-install manifests,  level=warning msg=failed to parse first occurrence of unknown field: failed to unmarshal install-config.yaml: error unmarshaling JSON: while decoding JSON: json: unknown field \"username\"\nlevel=info msg=Attempting to unmarshal while ignoring unknown keys because strict unmarshaling failed\nlevel=error msg=failed to fetch Master Machines: failed to load asset \"Install Config\": failed to create install config: invalid \"install-config.yaml\" file: [platform.vsphere.vcenters.username: Required value: m <TRUNCATED>: exit status 3" go-id=1569 pkg=Inventory request_id=