dvs_switch manifests can not be reapplied.

[nanliu]

Currently dvs_switch host need to specify operation create/edit/remove. This results in a manifest that can not be re-applied:

host => [
  {
    host => "${esx1['hostname']}",
    operation => 'add',
    backing => {
      pnicSpec => [
        {pnicDevice => 'vmnic1', uplinkPortgroupKey => 'dvs1-uplink-pg'},
      ],
    },
    maxProxySwitchPorts => 128,
  }
]

We should consider hiding the API internals so the resource can behave more appropriately.