mediawiki: use HHVM module on trusty

* Get rid of various $use_hhvm/$jobrunner_hhvm-style parameters. We can't run
  MediaWiki on Trusty without HHVM, so the distro version check is both
  necessary and sufficient for determining which interpreter we should use.
* For the same reason: include ::mediawiki::hhvm from ::mediawiki if >= Trusty.
* Get rid of mediawiki::jobrunner::hhvm. There is no longer anything unique
  about the jobrunner HHVM setup. (We don't want to disable the service either
  since we're transitioning to dispatching jobs via FastCGI).
* Lint: alphabetize modules in mediawiki::web::modules; fix indent; add
  trailing comma to last param; alphabetize lists.
* Get rid of duplicate max_req_workers in mediawiki::web::modules (the modules
  class already has to call lookupvar for getting other variables from
  mediawiki::web, so there's no reason to duplicate it).
* Drop mpm_worker.conf.erb; not in use and doesn't work. (Scope lookup
  failures.)
* Make mpm_prefork be an apache::conf resource.

Change-Id: I4aa104920d8121033e664458bb72c59233ef3868