Add support for the no_device setting for a block device in ec2.

I have an AMI that has both `/dev/sda` (root) and `/dev/sdf` defined against it.

When I run:

```
- ec2:
    image: XXXXX
    volumes:
          - device_name: /dev/sdf
            volume_size: 0
```

I expect the device to not be created or attached, but instead it still creates the device.

This patch fixes the problem by setting the `no_device` flag when creating the `BlockDeviceType` which correctly suppresses the device defined in the AMI and produces the expected behaviour.