prevent apache2 service log churn on non-web mediawiki servers

This fixes a bug I introduced in the patch series that culminated in
Ie0807c706, the result of which is the apache2 service getting picked up by the
non-web application servers.

* Rename mediawiki::service to mediawiki::web
* Move contents of mediawiki::config::apache to mediawiki::web
* ...with the exception of File['/etc/cluster'], which belongs in init.pp.
* Include mediawiki::web in role::mediawiki::webserver.

There is substantial clean-up work remaining: the whole mediawiki::config::*
hierarchy ought to be dissolved, with mediawiki::config::php moved up to
mediawiki::php, but this is a first step.

To validate this patch after it is merged:

* Run Puppet on mw1114 (api), mw1021 (app), mw1149 (bits) and confirm that the
  patch is a no-op on these host classes.

* Run Puppet on mw1157 (image scaler), mw1002 (job runner), and tmh1001 (video
  scaler) and confirm that the apache service is stopped and not restarted.

Change-Id: I0b2580a757fc1c54987a290d1d3aaf006f7f794e