providers/aws: rework instance block devices

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* AWS API
calls. They're only available from the instance meta-data service as
queried from inside the node.

Also note 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.

Fixes #858