Add CMake cache option for VTK_BUILD_PYTHON_MODULE_DIR

This property allow ParaView to override the default VTK
value ($BUILD/Wrapping/Python/) to be where ParaView
expect its Python modules to be build.

Change-Id: I5c2cc016356d7a4320f9f5c999225835abcd6bb1
Merge topic 'fix-python-cmake-rules' into master

14e73bd9 Add CMake cache option for VTK_BUILD_PYTHON_MODULE_DIR