govc: add storage.policy.create zonal option

[dougm]

New '-z' flag enables Zonal topology for multi-zone Supervisor

Uses the sub profile as created by the H5 UI, see for example:

govc storage.policy.info -json wcp-zonal | jq .policies[].profile.constraints.subProfiles

[
  {
    "name": "Consumption domain",
    "capability": [
      {
        "id": {
          "namespace": "com.vmware.storage.consumptiondomain",
          "id": "StorageTopology"
        },
        "constraint": [
          {
            "propertyInstance": [
              {
                "id": "StorageTopologyType",
                "value": "Zonal"
              }
            ]
          }
        ]
      }
    ]
  }
]