Improvements to mediawiki_singlenode

 * Change ServerAdmin to a URL, pointing at module docs.
   See <http://httpd.apache.org/docs/current/mod/core.html#serveradmin>.
 * In erb templates, use '@varname' to refer to Puppet vars; 'varname' can
   clash with Ruby names.
 * Rename 'simplewiki.wmflabs.org' to 'mediawiki_singlenode.erb'.
 * Redirect root URL to MediaWiki main page.

Change-Id: Iddae7ed1f61094e27a89b87049597f2ebd92a4e8