Deploying ceph-osd without ceph-cluster-network config set generated "Falling back to public interface" error message in Ceph

# Problem

Deploying ceph-osd with bindings for `public` and `cluster` defined results in the following error message for OSDs (for example /var/log/ceph/ceph-osd.0.log)

2022-03-25T18:00:05.060+0000 7f6eecde02c0 -1 Falling back to public interface

The error stems from:
https://github.com/ceph/ceph/blob/ee28fb57e47e9f88813e24bbf4c14496ca299d31/src/common/pick_address.cc#L411-L416

As far as I can tell when testing, unless the `cluster network` is set in /etc/ceph/ceph.conf (via `ceph-cluster-network` config flag), Ceph will fall back to just public interface.

So if you're only using the network space bindings as recommended in the ceph-osd charm documentation, it will not generate the correct ceph configuration file.

# Bundle
relations:
- - ceph-osd:mon
  - ceph-mon:osd
series: focal
applications:
  ceph-mon:
    charm: cs:ceph-mon-73
    num_units: 3
    constraints: tags=ceph-mon
    bindings:
      "": site2-oam
      public: site2-ceph-public
    options:
      monitor-count: 3
      source: cloud:focal-xena
  ceph-osd:
    charm: cs:ceph-osd-513
    num_units: 3
    constraints: tags=ceph-osd
    options:
      osd-devices: "/dev/sdb /dev/sdc /dev/sdd /dev/sde /dev/sdf /dev/sdg /dev/sdh /dev/sdi /dev/sdj /dev/sdk /dev/sdl /dev/sdm"
      source: cloud:focal-xena
    bindings:
      "": site2-oam
      public: site2-ceph-public
      cluster: site2-ceph-cluster

# juju spaces
Name Space ID Subnets
alpha 0
site2-os-public 1 10.42.208.0/23
site2-neutron-external 2 10.42.210.0/23
site2-os-data 3 172.17.100.0/23
site2-os-internal 4 172.17.102.0/23
site2-ceph-public 5 172.17.104.0/23
site2-ceph-cluster 6 172.17.106.0/23
site2-oam 7 10.42.200.0/23
site2-provision 8 10.42.196.0/23

# versions
juju 2.9.27
ceph-mon rev 73
ceph-osd rev 513
ceph 16.2.6