Ensure root device is listed in block device mappings

[nuwang]

No description provided.

[coveralls]

Coverage remained the same at 93.58% when pulling c116c57 on gvlproject:root_device_mapping into 44bd6e8 on spulec:master.

[JackDanger]

I'm not very familiar with this API but the Example #1 on this page shows a root device of /dev/sda and then a deviceName of /dev/sda1: https://docs.aws.amazon.com/AWSEC2/latest/APIReference/API_DescribeImages.html

[JackDanger]

We can merge this PR as-is because Example #2 on that page matches what your changes suggest. I just wonder if either the AWS docs are wrong (which happens) or if there's something else going on here.

[nuwang]

@JackDanger I suspect that the example may be wrong. The docs here state the following:
"Each AMI has a block device mapping that specifies the block devices to attach to an instance when it is launched from the AMI. An AMI that Amazon provides includes a root device only. To add more block devices to an AMI, you must create your own AMI."

The docs here say that "Every time you launch an instance from an AMI, a root storage device is created for that instance. The root storage device contains all the information necessary to boot the instance"

Putting the two things together, I think it's safe to assume that the root device always appears in block device mappings for the AMI, although I couldn't find an unambiguous statement stating this explicitly. It's also consistent with how things work through the console and APIs.

[JackDanger]

@nuwang Thanks for figuring this out! Once again Moto developers eclipse the AWS docs in accuracy 👍