Instance block devices are now managed by three distinct sub-resources:
* `root_block_device` - introduced previously
* `ebs_block_device` - all additional ebs-backed volumes
* `ephemeral_block_device` - instance store / ephemeral devices
The AWS API support around BlockDeviceMapping is pretty confusing. It's
a single collection type that supports these three members each of which
has different fields and different behavior.
My biggest hiccup came from the fact that Instance Store volumes do not
show up in any response BlockDeviceMapping for any EC2 `Describe*` API
calls. They're only available from the instance meta-data service as
queried from inside the node.
This removes `block_device` altogether for a clean break from old
configs. New configs will need to sort their `block_device`
declarations into the three new types. The field has been marked
`Removed` to indicate this to users.
With the new block device format being introduced, we need to ensure
Terraform is able to properly read statefiles written in the old format.
So we use the new `helper/schema` facility of "state migrations" to
transform statefiles in the old format to something that the current
version of the schema can use.
Fixes#858
Paul Hinze