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# Type of request: This is a ...

[x] bug report
[ ] feature request

# Detailed description
VTL request containing an expression evaluated to String Concatenation does not work.

### Query
```
mutation createShop {
            createShop(shop: { shopName: "Apple" } ) {
              shopName
            }
          }
```

### VTL Template (**Works when deploy to Appsync on AWS services**)
```
{
  "version" : "2018-05-29",
  "operation" : "PutItem",
  "key" : {
    "PK": $util.dynamodb.toDynamoDBJson("SHOP#${ctx.args.shop.shopName}"),
    "SK": $util.dynamodb.toDynamoDBJson("PROFILE#$ctx.args.shop.shopName")
  },
  "attributeValues" : { "shopName": { "S": "Apple" }}
}
```

## Expected behavior
### Item stored in dynamodb 
```
{
            "PK": {
                "S": "SHOP#Apple"
            },
            "SK": {
                "S": "PROFILE#Apple"
            },
            "shopName": {
                "S": "Apple"
            }
        }
```


## Actual behavior
### Item stored in dynamodb 
```
{
            "PK": {
                "S": "SHOP#${ctx.args.shop.shopName}"
            },
            "SK": {
                "S": "PROFILE#$ctx.args.shop.shopName"
            },
            "shopName": {
                "S": "Apple"
            }
        }
```

# Steps to reproduce

## Command used to start LocalStack
docker-compose
````
version: '2.1'

services:
  localstack:
    container_name: "${LOCALSTACK_DOCKER_NAME-localstack_main}"
    image: localstack/localstack
    ports:
      - "4566-4700:4566-4700"
      - "443:443"
      - "${PORT_WEB_UI-8080}:${PORT_WEB_UI-8080}"
    environment:
      - SERVICES=serverless,cloudformation,dynamodb,iam,s3,appsync,edge
      - DEBUG=1
      - DATA_DIR=${DATA_DIR- }
      - PORT_WEB_UI=${PORT_WEB_UI- }
      - LAMBDA_EXECUTOR=${LAMBDA_EXECUTOR- }
      - KINESIS_ERROR_PROBABILITY=${KINESIS_ERROR_PROBABILITY- }
      - DOCKER_HOST=unix:///var/run/docker.sock
      - HOST_TMP_FOLDER=${TMPDIR}
      - LOCALSTACK_API_KEY=${LOCALSTACK_API_KEY}
    volumes:
      - "${TMPDIR:-/tmp/localstack}:/tmp/localstack"
      - "/var/run/docker.sock:/var/run/docker.sock"
````

## Client code (AWS SDK code snippet, or sequence of "awslocal" commands)

...

config=None, verify=False, *args, **kwargs):
    """
    Generic method to obtain an AWS service client using boto3, based on environment, region, or custom endpoint_url.
    """
    region_name = region_name or get_region()
    env = get_environment(env, region_name=region_name)
    region = env.region if env.region != REGION_LOCAL else region_name
    key_elements = [service_name, client, env, region, endpoint_url, config]
    cache_key = '/'.join([str(k) for k in key_elements])
    if cache_key not in BOTO_CLIENTS_CACHE:
        
        my_session = get_boto3_session()
        method = my_session.client if client else my_session.resource
        if not endpoint_url:
            if is_local_env(env):
                endpoint_url = get_local_service_url(service_name)
                verify = False
        BOTO_CLIENTS_CACHE[cache_key] = method(service_name, region_name=region,
            endpoint_url=endpoint_url, verify=verify, config=config)
    return BOTO_CLIENTS_CACHE[cache_key]
class VelocityInput:
    """Simple class to mimick the behavior of variable '$input' in AWS API Gateway integration velocity templates.
    See: http://docs.aws.amazon.com/apigateway/latest/developerguide/api-gateway-mapping-template-reference.html"""
    def __init__(self, value):
        self.value = value
    def path(self, path):