Add image configurations for root device properties.
Add image configurations for root device properties.

The root device is `/dev/sda1` for Ubuntu AMIs, but can be otherwise for
other distributions. Support overriding name and size of the root device
from the `images` definition.

- `root_name`: The root device name, defaulting to `/dev/sda1`.
- `root_size`: The size to give the root device in GB. Defaults to snapshot size.

nth10sd/laniakea,MozillaSecurity/laniakea,MozillaSecurity/laniakea,nth10sd/laniakea