rbd-mirror: fix the below bugs

rbd-mirror: fix the below bugs

* fix braces in the imageMap update_images_added & update_images_removed
* do not allow image add from non-primary
* `down+unknown` status shown on querying individual images which are part
  of group enabled for mirroring
* `mirror pool status` shows down+unknown status
* fix imageMap being overwritten when multiple images are enabled for mirroring
* fix misleading error msg when getting status of a non-mirror enabled group

Signed-off-by: Prasanna Kumar Kalever <prasanna.kalever@redhat.com>