block_device_mapping support

Support the block_device_mapping attribute when creating a new ec2 instance.

Example:

```ruby 
aws.block_device_mapping = [
    {
        :DeviceName => "/dev/sdb", 
        :VirtualName => "ephemeral0"
    },
    {
        :DeviceName => "/dev/sdc", 
        :VirtualName => "ephemeral1"
    }
]
```