BUG: Update SlicerExecutionModel - Fixes #2712

This commit updates SlicerExecutionModel to include an updated version
of ModuleDescription::ReadParameterFile function that support string
parameter without value. The Slicer/SlicerExecutionModel@05af78d also
adds corresponding test checking that parameter file with and without
value can still be read.

$ git shortlog feef59e..a6616b52 --no-merges
Jean-Christophe Fillion-Robin (1):
      BUG: Support parameter file with empty value

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23478 3bd1e089-480b-0410-8dfb-8563597acbee