mon: add a unique clsuter fingerprint

The fsid is intended to be unique, but because you can provide it to mkfs,
it is very easy to intentionally or unintentionally share it across
clusters.  This makes it useless as a unique identifier for a cluster
instance.

We can't remove the ability to provide the fsid during mkfs because it would
break existing tools and because many valid workflows require it.

Introduce a new identifier, the 'fingerprint', that is generated *only* by
the mons themselves during cluster creations or after an upgrade.  Include
this in the mon report to serve as an identifier.