stack/erp5: SOFTWARE_HOME was incorrectly set to Zope2.egg/Zope2.

As SOFTWARE_HOME is added at the top of sys.path, this meant that Zope2.egg/Zope2
ended up at the top of sys.path:
  * This broke `imp.find_module('App')` and Pylint (`No name 'Extensions' in
    module 'App' (no-name-in-module)`) because there are `App` modules in both
    Zope2.egg/Zope2/ and Zope2.egg/ and the former was returned.
  * "Normal" instances and Products.ERP5Type.tests.runUnitTest properly set up
    SOFTWARE_HOME to Zope2.egg/.

Additionally, according to Zope2/Testing documentation, SOFTWARE_HOME is only
needed to find Zope2.egg/Testing module, already available as Zope2.egg is in
sys.path.