The RADOS Block Device (rbd) can be used as any other block device with
further layers on top of it, hence allow the common persistent storage
rules to apply, including watching for changes.
This change is only about the source tree. We have tmpfiles.d/, modprobe.d/,
sysctl.d/, and sysusers.d/, but for historical reasons, rules/ didn't fit this
pattern. We also *install* it as rules.d/. Let's rename to be consistent.