tests: Added util functions

Added functions to extract gfid, gfid-str
function to check if a file is open
deleting gfid-link of a file

Change-Id: If2f39f43a6631cddb68b4ba7febcd3cf66f399ee
BUG: 821056
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/4386
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>