mon/ConfigKeyService: dump: print placeholder value for binary blobs

mon/ConfigKeyService: dump: print placeholder value for binary blobs

JSON cannot express arbitrary binary blobs.  Instead of outputting invalid
and unparseable JSON, represent the value of blobs as something like
'<<< binary blob of length 12 >>>'.

Fixes: http://tracker.ceph.com/issues/23622
Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit 4669134b89ee1f7759fb59faef218da0880daa98)

Conflicts:
	PendingReleaseNotes: 12.2.6 dropped, will be added in a seperate pr
	src/mon/ConfigKeyService.cc (ConfigKeyService::store_dump does not take
            prefix argument in luminous)