COMP: DashboardDriver: Introduce SVN_REPOSITORY variable

By default the variable is set to:

  http://svn.slicer.org/${CTEST_PROJECT_NAME}

If needed, the variable SVN_REPOSITORY can also be set prior to include
the driver script.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22842 3bd1e089-480b-0410-8dfb-8563597acbee