/cc @vpelletier @jm @rafael

without this change, we have this error:

[2019-05-07 12:33:46,742] INFO     Installing dynamic-template-balancer.
[2019-05-07 12:33:46,742] INFO     While:
[2019-05-07 12:33:46,742] INFO       Installing dynamic-template-balancer.
[2019-05-07 12:33:46,742] INFO     
[2019-05-07 12:33:46,742] INFO     An internal error occurred due to a bug in either zc.buildout or in a
[2019-05-07 12:33:46,743] INFO     recipe being used:
[2019-05-07 12:33:46,743] INFO     Traceback (most recent call last):
[2019-05-07 12:33:46,743] INFO       File "eggs/zc.buildout-2.5.2+slapos014-py2.7.egg/zc/buildout/buildout.py", line 2243, in main
[2019-05-07 12:33:46,743] INFO         getattr(buildout, command)(args)
[2019-05-07 12:33:46,743] INFO       File "eggs/zc.buildout-2.5.2+slapos014-py2.7.egg/zc/buildout/buildout.py", line 639, in install
[2019-05-07 12:33:46,743] INFO         self._install_parts(install_args)
[2019-05-07 12:33:46,743] INFO       File "eggs/zc.buildout-2.5.2+slapos014-py2.7.egg/zc/buildout/buildout.py", line 808, in _install_parts
[2019-05-07 12:33:46,743] INFO         installed_files = self[part]._call(recipe.install)
[2019-05-07 12:33:46,743] INFO       File "eggs/zc.buildout-2.5.2+slapos014-py2.7.egg/zc/buildout/buildout.py", line 1607, in _call
[2019-05-07 12:33:46,743] INFO         return f()
[2019-05-07 12:33:46,744] INFO       File "eggs/slapos.recipe.template-4.3-py2.7.egg/slapos/recipe/template/jinja2_template.py", line 237, in install
[2019-05-07 12:33:46,744] INFO         rendered = template_object.render(**self.context).encode(self.encoding)
[2019-05-07 12:33:46,744] INFO       File "eggs/Jinja2-2.9.5-py2.7.egg/jinja2/environment.py", line 1008, in render
[2019-05-07 12:33:46,744] INFO         return self.environment.handle_exception(exc_info, True)
[2019-05-07 12:33:46,744] INFO       File "eggs/Jinja2-2.9.5-py2.7.egg/jinja2/environment.py", line 780, in handle_exception
[2019-05-07 12:33:46,744] INFO         reraise(exc_type, exc_value, tb)
[2019-05-07 12:33:46,744] INFO       File "parts/template-balancer/instance-balancer.cfg.in", line 86, in top-level template code
[2019-05-07 12:33:46,744] INFO         {%   do zope_family_address_list[0][0] -%}
[2019-05-07 12:33:46,744] INFO       File "eggs/Jinja2-2.9.5-py2.7.egg/jinja2/environment.py", line 411, in getitem
[2019-05-07 12:33:46,744] INFO         return obj[argument]
[2019-05-07 12:33:46,745] INFO     UndefinedError: list object has no element 0

because in instance-balancer.cfg we are accessing zope_family_address_list[0][0] but this variable is empty. It is constructed in https://lab.nexedi.com/nexedi/slapos/blob/master/stack/erp5/instance-balancer.cfg.in#L57