wic: fix errors in partition.py module

wic: fix errors in partition.py module

Fixed the followring errors found by pylint in partition.py:

E: 42, 0: class already defined line 33 (function-redefined)
E:429,63: Undefined variable 'fs' (undefined-variable)
E:432,37: Undefined variable 'fs' (undefined-variable)
E:481, 0: class already defined line 33 (function-redefined)
E:488,22: Undefined variable 'OptionValueError' (undefined-variable)

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>