apache: add a 'replaces' parameter to apache::conf

This will allow us to remove, whenever we feel like we need to, to
ensure another file (tipically a module config file) shipped by the
distro will not be included in the apache config.

Change-Id: I137a5d40a669ca651a039c98bcbf5ba1b08d286e
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>