Modify build to allow external projects to change VTK repo.
Created by: tjcorona
This commit is in reference to Issue # 4148. The default VTK repository and tag are now force set and are used as a fallback if 'Slicer_VTKv6_GIT_REPOSITORY' or 'Slicer_VTKv6_GIT_TAG' are not already set in the project.