mediawiki: use pidfile from the env vars

Apache envvars are set so that ${APACHE_PID_FILE} is an environment
variable that would be used by the init scripts and apache2ctl; on the
contrary, apache2 itself will read that value from the PidFile directive
in its configuration. If the two values are not identical, apache will
become unable to reload and in general manage itself. This change should
fix the current situation in prod, where most appservers are unable to
gracefully reload due to a discrepancy in this value.

Change-Id: I937d645864c10ac24a7ae25143ae1b9140aac706
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>