cluster/afr: Add dry-run functionality to self-heal.

This will be useful in figuring out if a file needs
self-heal or not with certainity for data-self-heal

Change-Id: Idf98a68e69f2c35646ef2e7c97302586fe1dc07d
BUG: 1039544
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/6510
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Ravishankar N <ravishankar@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>