Background value for VectorDataToLabelImageFilter

I made a few little corrections in the otbVectorDataToLabelImageFilter:

• Fix a typo (woah!),
• Add a m_BackgroundValue (plus its itkSetMacro/itkGetMacro): usefull when you want a specific value for pixels where no geometries are burnt,
• Add an itkSetMacro/itkGetMacro for the already implemented m_DefaultBurnValue