add-disk action is missing option to specify which BlueStore WAL/DB block device will be used

[Sandor Zeestraten]

# Problem
When adding OSDs with the add-disk action, it is not possible to specify which BlueStore WAL/DB device (`bluestore-db`) will be used if there are multiple WAL/DB devices.

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

# 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-public-network: "172.17.104.0/23"
  ceph-osd:
    charm: cs:ceph-osd-513
    num_units: 3
    constraints: tags=ceph-osd
    options:
      osd-devices: "/dev/sda /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
      ceph-public-network: "172.17.104.0/23"
      ceph-cluster-network: "172.17.106.0/23"
      bluestore-db: "/dev/nvme0n1 /dev/nvme1n1"
      bluestore-block-db-size: "265000000000"
    bindings:
      "": site2-oam
      public: site2-ceph-public
      cluster: site2-ceph-cluster