### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current Behavior

Currently the VTL template for `request_templates` is not processed properly:

```
#set( $body = $input.json("$") )

#macro( loop $map )
{
    #foreach($key in $map.keySet())
        #set( $k = $util.escapeJavaScript($key) )
        #set( $v = $util.escapeJavaScript($map.get($key)).replaceAll("\\'", "'") )
        "$k": "$v"
        #if( $foreach.hasNext ) , #end
    #end
}
#end

{
    "enhancedAuthContext": #loop($context.authorizer),

    "headers": #loop($input.params().header),

    "query": #loop($input.params().querystring),

    "path": #loop($input.params().path),

    "identity": #loop($context.identity),

    "stageVariables": #loop($stageVariables),
}
EOF
}
```

results in,

```
'

{
    "enhancedAuthContext": {
    }
,

    "headers": {
    }
,

    "query": {
    }
,

    "path": {
    }
,

    "identity": {
    }
,

    "stageVariables": {
    }
,
}
'
```

### Expected Behavior

The template should be rendered and have the JSON properties.

### How are you starting LocalStack?

Custom (please describe below)

### Steps To Reproduce

#### How are you starting localstack (e.g., `bin/localstack` command, arguments, or `docker-compose.yml`)

    docker run localstack/localstack

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

    awslocal s3 mb s3://mybucket


### Environment

```markdown
- OS: MacOS
- LocalStack: 0.14
```


### Anything else?

This example can be used to validate.
https://github.com/localstack/localstack-terraform-samples/tree/master/apigateway-lambda-velocity
"""Simple class to mimick the behavior of variable '$input' in AWS API Gateway integration velocity templates.
    See: http://docs.aws.amazon.com/apigateway/latest/developerguide/api-gateway-mapping-template-reference.html"""
    def __init__(self, value):
        self.value = self._attach_missing_functions(value)
    def path(self, path):
        if not self.value:
            return {}
        value = self.value if isinstance(self.value, dict) else json.loads(self.value)
        return extract_jsonpath(value, path)
    def json(self, path):
        path = path or "$"
        matching = self.path(path)
        if isinstance(matching, (list, dict)):
            matching = json_safe(matching)
        return json.dumps(matching)
    def __getattr__(self, name):
        return self.value.get(name)
    def __repr__(self):
        return "$input"
    def _attach_missing_functions(self, value):
        if value:
            def _fix(obj, **kwargs):
                if isinstance(obj, dict):
                    return DictWrapper(obj)
                return obj
            value = recurse_object(value, _fix)
        return value
class VelocityUtil(object):