Fix OTB_ADDITIONAL_CACHE

In the SuperBuild the OTB_ADDITIONAL_CACHE variable can be used to pass options to the OTB project. However the description of this variable is written on several lines. Instead of being empty by default, the variable is initialized to CACHE;STRING;Additional cmake options for OTB with ;the syntax (semi-colon used as a separator):-D<var>:<type>=<value>;-D<var>:<type>=<value>, which is the interpreted as the value for the previous variable in otb project, i.e. the Python path :

ExternalProject_Add(OTB
  DEPENDS ${OTB_DEPENDENCIES}
  PREFIX OTB
  DOWNLOAD_COMMAND ""
  SOURCE_DIR ${OTB_SB_SRC}
  BINARY_DIR ${OTB_SB_BUILD_DIR}
             [...]
  -DOTB_USE_MPI:BOOL=${OTB_USE_MPI}
  -DOTB_USE_SPTW:BOOL=${OTB_USE_SPTW}
  -DPYTHON_EXECUTABLE:PATH=${PYTHON_EXECUTABLE}
  ${OTB_ADDITIONAL_CACHE}
  CMAKE_ARGS ${OTB_SB_CONFIG}
  CMAKE_COMMAND ${SB_CMAKE_COMMAND}
  LOG_CONFIGURE 1
  )

resulting in:

PYTHON_EXECUTABLE=/usr/bin/python3.6;CACHE;STRING;Additional cmake options for OTB with ;the syntax (semi-colon used as a separator):

This is the cause of bug described in #1900 (closed)