LBCORE-921 First version of alternatives manager dealing with python

First version.

The alternatives/python directories contains links to all the site-packages directories (not individual tools). LbEnv.sh scans the directory and adds all subdirs to the path. If a link target is changed, the new code is immediately visible, but if a new subdir is created, it is NOT in the path (until LbEnv.sh is sourced again).