object-storage: Use the wrapper functions provided by fs_utils.py to make
  system calls.

 The set of changes:
 * Unit test cases for fs_utils.py

 * Replaced os.path with os_path

 * Implemented wrapper functions do_write, do_chmod, etc in fs_utils.py

 * Replaced os.<sys-call> with the wrapper functions.

Change-Id: I770da878e83eda6b98e49d70193990406a2642a7
BUG: 887301
Signed-off-by: Mohammed Junaid <junaid@redhat.com>
Reviewed-on: http://review.gluster.org/4360
Reviewed-by: Peter Portante <pportant@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>