provider/openstack: Convert block_device from TypeSet to TypeList

[jtopjian]

This change better reflects how block devices are passed to the Nova API
and allows for future enablement of block_device features. It also resolves
an interpolation bug.

Fixes #4090

[jtopjian]

I'm going to to go ahead and merge this one. Although I think the real cause of #4090 is a parsing bug in core, rather than fixing this issue by that route, I think changing block_device to TypeMap does three things:

1. Inadvertently fixes the problem reported in openstack_compute_instance_v2.master: diffs didn't match during apply. This is a bug with Terraform and should be reported. #4090
2. Better models how the block devices are submitted via the OpenStack API (See Adding multiple Ephemeral Volumes to Compute Instance on OpenStack #4042 as well as numerous references in the OpenStack code that ensure block_device_mapping_v2 is a list (Python array)).
3. Enables features such as Adding multiple Ephemeral Volumes to Compute Instance on OpenStack #4042 to be enabled.

May I eat a hat if this blows up on me.

[ghost]

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