mediawiki: add envvars class, get rid of cleaning directive.

we need /var/lock/apache2 to be owned by apache or apache2ctl will
complain and not restart apache.

Also, I grouped envvars-related resources in a separate class as I
suspect it will become larger.

Change-Id: Ia40cc8c026dbd36d346f7e39526b445cfc7a9f87