WSGI script: bring some sanity

 - avoid hardcoding Python version;

 - allow to specify multiple directories inside WSGI_PATH
   with usual Unix separator (':');

 - don't use default values for virtual environment and
   WSGI path: first one is always requred, so we bail out
   if it isn't specified; the second one is optional, so
   we will just notify about its absence;

 - rearrange blocks to group same-purpose code;

 - remove some old commented cruft.

Signed-off-by: Eygene Ryabinkin <rea@grid.kiae.ru>