Updating the block device options for ASGs

There are more options available for block device launch configs, so I have updated them.

This also allows you to take advantage of the new General Purpose SSDs by doing this:

```python
launchConfig = t.add_resource(asg.LaunchConfiguration(
    "launchConfig",
    BlockDeviceMappings=[asg.BlockDeviceMapping(
        DeviceName="/dev/sda1",
        Ebs=asg.EBSBlockDevice(
            VolumeType="gp2"
        )
    )],
   ...
))
```

Tested this and it works in `eu-west-1`