CBQE-0: MB-9707: Updating this code from fail to warning to avoid test
to abort, as per this bug, this particular stat i.e.
replication_changes_left is buggy

Change-Id: I9d6edb99c97ee39100e2c58cecb692472a759c46
Reviewed-on: http://review.couchbase.org/32662
Reviewed-by: Sangharsh Agarwal <sangharsh.agarwal@globallogic.com>
Tested-by: Sangharsh Agarwal <sangharsh.agarwal@globallogic.com>