If the cluster has 3 OSDs, during the upgrade when the OSD restarts it
will temporarily disappear from the "ceph versions" list so we need a
more robust way to list the number of OSDs and always returns the total
number of OSDs.
Closes: https://github.com/rook/rook/issues/5521
Signed-off-by: Sébastien Han <seb@redhat.com>