mon: add a cluster fingerprint

Generate it on cluster creations with the initial monmap.  Include it in
the report.  Provide no way for this uuid to be fed in to the cluster
(intentionally or not) so that it can be assumed to be a truly unique
identifier for the cluster.

Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit 675b0042eff0ad5e1453838410210b1206c39004)
mon: generate cluster_fingerprint if null

This triggers after an upgrade of a legacy cluster that has no fingerprint.

Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit b245d600163f6337af15aedd1fea68f4e2a668a8)