Fix-ups for Iddc778a28

* mediawiki: Dedupe libapache2-mod-php5
  Conflicts with apache::mod::php5, which is the proper means of
  grabbing that package.
* Don't declare the site-enabled file *and* an identical symlink
  in webserver::apache::site.

Change-Id: Ib6df135f75de6ffe3e4e12e914a5b597d4a4fd6c