Some AMIs have a RootDeviceName like "/dev/sda1" that does not appear as a
DeviceName in the BlockDeviceMapping list (which will instead have
something like "/dev/sda")
While this seems like it breaks an invariant of AMIs, it ends up working
on the AWS side, and AMIs like this are common enough that we need to
special case it so Terraform does the right thing.
Our heuristic is: if the RootDeviceName does not appear in the
BlockDeviceMapping, assume that the DeviceName of the first
BlockDeviceMapping entry serves as the root device.
fixes#2224
020dc03234