New block_device_mappings config

[tyler-ball]

This PR rolls a bunch of existing, storage based PRs into one:

• Closes Can't find how to set EBS Volume Size #71
• Connects to Add root device size updated #64
• Connects to Add root device size option #43
• Connects to Added config option and method to allow specifying block device mappings #78
• Connects to Allow EBS volume type to be specified. #76

I am deprecating the old, individual storage config keys and adding a new one which accepts a list. This allows users to specify multiple block device mappings. If one of the entries has the same name as the image root storage device it will be overridden.

To support backwards compatiability if you do not use the new block_device_mappings config it will be auto created using the old deprecated configs. It emits a warning because we will eventually remove this functionality.

How are the READMEs? What do you think about usability?

\cc @fnichol @rvalyi @victorlin @nsdavidson @duggan

[tyler-ball]

Okay tests have been added - this is ready for review!