ENH: Support execution of python binaries using SlicerPython launcher env

Running the python executable (or any binaries installed by python) using
SlicerPython launcher environment will now resolve to the correct binary.

For example, here are after/before results associated with a linux Slicer
build tree.

After:

$ eval $(SlicerPython --launcher-show-set-environment-commands)
$ which python
/path/to/python-install/bin/python

Before:

$ eval $(SlicerPython --launcher-show-set-environment-commands)
$ which python
/usr/bin/python

Note that within a Slicer package, python executable is currently
renamed to "python-real". See r23057 (BUG: Rename installed python
executable to account for case-insistive system) for the rational.

Background:

The python project built by Slicer is installed in
/path/to/Slicer-SuperBuild/python-install, in addition to the python executable,
a launcher named "SlicerPython" is also configured and set as PYTHON_EXECUTABLE
CMake variable to ensure build step relying on python built by Slicer resolve
to the expected environment.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25954 3bd1e089-480b-0410-8dfb-8563597acbee