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: Id1aaf661de25f0c688a34a3c6d747f30283deb24
Merge topic 'remove_python_interpretor_warning-rel' into master

99d62fbb Get rid of warning when using VTK externally through Python.