auth/cephx: tolerate missing rotating keys

auth/cephx: tolerate missing rotating keys

During an upgrade, we may have a client requesting an
MGR service key but not have one in the database yet,
either because we *just* upgraded and haven't generated
one yet, or because the leader mon hasn't been upgraded
yet.

Fix this by silently tolerating a missing key as long as
one or more other service keys were present and we have
something to give to the client.

Signed-off-by: Sage Weil <sage@redhat.com>