Adapt instance skeleton for egg cache

[gracinet]

The startup scripts (runzope, zopectl) should set the PYTHON_EGG_CACHE environement var. This is necessary, otherwise the instance won't start if python-ldap is being used.

Now question : how to create in a clean manner the corresponding directory with either zope 2.9 or zope 2.10 packages ? Both packages could be installed on the same system

Of course each instance could have its cache, but that's a waste of resources.