Introduce buildout:zcml-additional-fragments buildout variable.

The buildout:zcml-additional-fragments variable helps wrapping the instanceX:zcml-additional
option with a root configuration node, which allows to append to the
buildout:zcml-additional-fragments any ZCML which will then properly set up.

For more information regarding the source of the problem take a look at:
https://github.com/plone/plone.recipe.zope2instance/pull/13