Fix to resolve VPC NAT routing issue https://github.com/ansible/ansible/issues/7368

According to [boto.vpc.VPCCOnnection.create_route](http://bit.ly/1sku9W6) there is a need to use named parameters in the method call to differentiate between a gateway ID and an instance ID when trying to create a route.

there is a need to use named parameters in the method call
to differentiate between a gateway ID and an instance ID when
trying to create a route

Otherwise upon trying to register a route with an existing instance ID
instead of the gateway ID one gets an error like follows:

    failed: [127.0.0.1] => {"failed": true, "item": ""}
    msg: Unable to create and associate route table {u'routes': [{u'dest': u'0.0.0.0/0', u'gw': u'i-09d47501'}], u'subnets': [u'10.1.10.0/24', u'10.1.12.0/24', u'10.1.20.0/24']}, error: EC2ResponseError: 400 Bad Request
    <?xml version="1.0" encoding="UTF-8"?>
    <Response><Errors><Error><Code>InvalidGatewayID.NotFound</Code><Message>The gateway ID 'i-09d47501' does not exist</Message></Error></Errors><RequestID>fa95104e-37ab-44ee-b8fd-2c5033179585</RequestID></Response>