flexibilize zope2instance site.zcml

[Wolfgang Schnerring]

I've filed this here first: https://dev.plone.org/plone/ticket/8978, but it seems that was the wrong bug tracker. Sorry.

The plone.recipe.zope2instance only accepts ZCML files for inclusion into site.zcml that are called "configure", "meta" or "overrides" -- but sometimes you want to include files that have different names (say, ftesting.zcml).

The attached patch adds a little flexibility, so that a file called foo.zcml will be included by a snippet named package-foo-configure.zcml (which is then picked up by Five's default site.zcml).