Get rid of warning when using VTK externally through Python.

When importing VTK through Python without running vtkpyython,
Python will already be initialized. This gives a warning for
that but it should be taken out. This commit gets rid of that
undesired warning.

Change-Id: Id73316ace7dbd605eca770185c3b83c414de1357
Merge topic 'remove_python_interpretor_warning' into master

87bd9563 Get rid of warning when using VTK externally through Python.