mediawiki: HAT appserver should turn off mod_php

Apparently, installation on php-fss causes installation of
libapache2-mod-php, which will run a2enmod by default. We don't want to
waste memory and processing time to load this module as it's completely useless.

Change-Id: I0ba331cd8b5e4d47d429d89123102f767a7dea32
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>