Instance delete confirm modal: say disks will be detached but not deleted

[david-crespo]

1. Say "instance db1", we do that elsewhere and I think it's nice
2. Say something like "Attached disks will be detached, but they are not automatically deleted."
3. We could list the disks right in the modal, but it would require an additional fetch and I think there are rapidly diminishing returns to figuring out how to display some arbitrarily long list of disk names
4. We could theoretically mention other things that will or won't be deleted: ephemeral IPs will, NICs will, floating IPs won't. Bit of a design question to jam all that into the modal, but I think it could be helpful.