Parse `DescribeLaunchConfigurations` results correctly for Block Device Mappings

Fixes Issue #2048

* Removes redundant `BlockDeviceMapping` class
* Removes redundant `EBS` class
* Parse `<BlockDeviceMapping>` tag with existing class instead of `ResultSet`
* Add unit test for `DescribeLaunchConfig` with non-empty block device mappings (can probably be merged with original test)
* Allow `BlockDeviceMapping/Type` XML parsers to accept both EC2 instance XML blocks and Launch config blocks (mostly a capitalization issue)