Provide default mode for Volume.mkdir()

Calling mkdir without mode used to fail with following error:

>>> vol.mkdir('/c2')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  TypeError: mkdir() takes exactly 3 arguments (2 given)

Python's os.mkdir() defined default mode:
https://docs.python.org/2/library/os.html#os.mkdir

Also, changed mode_t is of type unsigned short (ctypes.c_ushort).

Change-Id: I95e47a8173d90e2a162cfeb9af002ea0c20fc2ff
Signed-off-by: Prashanth Pai <ppai@redhat.com>