rbd_mirror: order the user group snaps and the mirror group snaps syncing

rbd_mirror: order the user group snaps and the mirror group snaps syncing

In case of user group snap addition:
* Wait for the user group snap to sync to complete and only then mark
  the mirror group snap to complete
In case of user group snap removal:
* Wait for the user group snap to get pruned and only then mark the
  mirror group snap to complete

Updated tests:
  • test_create_group_with_images_then_mirror_with_regular_snapshots
      enable scenario 1 as with the recent changes it works, also remove
      additional mirror group snapshot as it is no more required.

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