extras: geo-rep: Customize the generate gfid script to dirs list

generate-gfid-file.sh now accepts dirs list file and generate
gfids only for those dirs list.

Change-Id: Ia78a0a744dc8a079db56c38578cc6fcac2a6fa90
Signed-off-by: Aravinda VK <avishwan@redhat.com>
Reviewed-on: http://review.gluster.org/8540
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>