message: Add more enum values for Identity/type in generic resource swagger
  * Add more enum values for Identity/type in generic resource swagger
  Resource types like "Microsoft.Compute/VirtualMachines" have more than
  one enum value for Identity/type. Updating generic resource swagger so
  it is up-to-date.
  * Revert changes in swagger spec for 2017 API Version
diff: "type": "string",
  "description": "The identity type.",
  "enum": [
  - "SystemAssigned"
  + "SystemAssigned",
  + "UserAssigned",
  + "SystemAssigned, UserAssigned",
  + "None"
  ],
  "x-ms-enum": {
  "name": "ResourceIdentityType",
  