cli: Throw a warning during replace-brick

Change-Id: Iae59365f09bf64a5927edeeb4c3c052e237eee38
BUG: 1039954
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/6560
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>