Fixed issue with VTK_WRAP_PYTHON not being set correctly.

We lost the logic to set VTK_WRAP_PYTHON correctly when PARAVIEW_ENABLE_PYTHON
in when enabling support for external VTK. This patch addresses that issue.

Change-Id: If7abaaabdf72217a13308f56a842a48c58e76805
Merge topic 'use_external_vtk' into next

b73e12d Fixed issue with VTK_WRAP_PYTHON not being set correctly.