"type" has to be singular by flattener code but can be an array by schema validation

This is to track if we should support multiple types or fix the code in cloudformation-cli to support multiple types. Right now the flatenner will fail when doing the following. Additionally the schema allows for multiple types.
```
{
"type": ["array", "object"],
"properties": {
"a": {}
},
"items": {
"properties": {
"b": {}
}
}
}
```

https://github.com/aws-cloudformation/cloudformation-cli/blob/f0e376989c1b5a106c97b25b78e98f46bdb5995f/src/rpdk/core/jsonutils/flattener.py#L19-L23

Schema definition: https://github.com/aws-cloudformation/cloudformation-resource-schema/blob/master/src/main/resources/schema/schema#L145-L155

```json
{
"type": {
"anyOf": [
{ "$ref": "#/definitions/simpleTypes" },
{
"type": "array",
"items": { "$ref": "#/definitions/simpleTypes" },
"minItems": 1,
"uniqueItems": true
}
]
}
}
```