Reinstall an instance

Rebuilds the server from a marketplace offering. The boot disk is erased. Requires confirm=true, typed_confirm set to the instance id, and acknowledge_unrecoverable=true. Needs instances:reinstall, which instances:write does not imply. A reinstall that would replace a recovery-critical secret cannot be made here -- use the dashboard, where each secret is shown and acknowledged. One that keeps every recovery-critical value (a customer-managed recovery key, for example) is accepted.