Resolved an issue with Instance#block_device_mappings and ephemeral volumes.

AWS::EC2::Instance#block_device_mappings was raising runtime errors if the
instance had any ephemeral volumes.  These are now filtered.  The method has
also be renamed to Instance#attachments (aliased to the old name).

Added Instance#block_devices which now returns all of the block devices
as a simple array of hashes.