Comment 3 for bug 1501435

This is actually a python-novaclient bug, it was introduced in this change - https://review.openstack.org/#/c/221525/

That removed block_device_mapping={"vda": "$volume_id"} support by eliminating the ==1 branch on _parse_block_device_mapping.

It was assumed safe because it was a private method, but that private method is doing data transformation on a value without any interpolation otherwise. So it's really part of the public contract.