test: utility to create files with a specific gfid

python test utility that uses setxattr() interface to
create files with a predefined gfid (which is also used
by geo-replication). this utility aids in writing test
cases for gfid-access translator.

Change-Id: Ib11248844a094cc02dae74d969f2e9212f94ede4
BUG: 1111490
Signed-off-by: Venky Shankar <vshankar@redhat.com>
Reviewed-on: http://review.gluster.org/8275
Reviewed-by: Aravinda VK <avishwan@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>