* provider/openstack: Redesign openstack_blockstorage_volume_attach_v2
The current design of openstack_blockstorage_volume_attach_v2 does
not correctly implement the Block Storage API attachment call. It
was only partially implemented, only marking volumes as being
attached, while never actually attaching them.
This redesign is a closer alignment to how creating attachments
to a standalone Block Storage service works.
For creating attachments specifically in the case of OpenStack
Compute instances, the openstack_compute_volume_attach_v2 resource
is required.
* provider/openstack: re-adding instance_id for backwards compatibility
This commit adds the openstack_blockstorage_volume_attach_v2 resource. This
resource enables a volume to be attached to an instance by using the OpenStack
Block Storage (Cinder) v2 API.