Updating code to call fs_utils instead of python's os module

This change is being done to prepare the code to always call
fs_utils for all filesystem calls on the data being stored.
By creating this interface (i.e., fs_utils), we can then
make a seamless change to use the current method of 'os.'
calls over FUSE _or_ libgfapi. This new method will be introduced
in a new separate patch.

Change-Id: Ic768fa7352d7672b1f060230bb4486f0ec228152
Signed-off-by: Thiago da Silva <thiago@redhat.com>
Reviewed-on: http://review.gluster.org/7333
Reviewed-by: Prashanth Pai <ppai@redhat.com>
Reviewed-by: Luis Pabon <lpabon@redhat.com>
Tested-by: Luis Pabon <lpabon@redhat.com>