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>
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>
Merge pull request #2291 from ceph/wip-fingerprint

mon: add a unique clsuter fingerprint

Reviewed-by: Joao Eduardo Luis <joao.luis@inktank.com>