fuse: make fuse parse environment variable to fetch new GFID

operations like

bash# GLUSTERFS_GFID=<gfid-of-new-file> touch /mnt/glusterfs/.gfid-path/<par-gfid>/testfile

will create 'testfile' under par-gfid path, with <gfid-of-new-file>.

Change-Id: I5659cf2ac1c7b883ff6955902e174bd7cbb93902
Signed-off-by: Amar Tumballi <amarts@redhat.com>