Allow creating tofu-controlled compute volumes

[sjpb]

Adds "volumes" key to compute module to define optional tofu-controlled volumes attached to compute nodes. Useful for simulating additional disks present in baremetal instances using virtualized instances.

TODO: test that, at least with no volumes actually defined, changing the image results in a "update" (i.e. rebuild) not delete/recreate. A delete/recreate is acceptable when:

• Changing to this TF for the first time
• Updating the image on a compute node with attached volumes (although it'd be nice to test/know whether it will do that)
• Adding volumes to a compute node which doesn't have them (will definitely be delete/recreate, no need to test)

[wtripp180901]

Warning: updating images for compute nodes currently forces them to be destroyed and replaced as the uuid of their block devices changes, even if no volumes are defined for them