rbd: extend use of "none" placeholder to IMAGES section

rbd: extend use of "none" placeholder to IMAGES section

Currently if there are no mirror-enabled images, IMAGES section
in "rbd mirror pool status --verbose" output isn't terminated:

  $ rbd mirror pool status data --verbose
  health: OK
  daemon health: OK
  image health: OK
  images: 0 total

  DAEMONS
  service 4388:
    ...
    health: OK

  IMAGES$

DAEMONS section has a "none" placeholder for when there are no
rbd-mirror daemons running.  Fix some issues with the separator logic
and employ the placeholder in IMAGES section:

  $ rbd mirror pool status data --verbose
  health: OK
  daemon health: OK
  image health: OK
  images: 0 total

  DAEMONS
  service 4388:
    ...
    health: OK

  IMAGES
    none
  $

  $ rbd mirror pool status data --verbose
  health: UNKNOWN
  daemon health: UNKNOWN
  image health: OK
  images: 0 total

  DAEMONS
    none

  IMAGES
    none
  $

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>