Fixes for Trusty

* Make sure /var/www and /etc/apache2/conf.d exist.
* /etc/apache2/sites-{enabled,disabled,available} files have '.conf' extension.
* uses php5enmod / php5dismod to enable/disable ini files for all SAPIs
  (php5 no longer has a cross-SAPI conf.d file; each config is SAPI-specific.)
* Use 'pecl list' to determine if xdebug is installed.

To test, edit Vagrant and s/precise/trusty/g.

With this patch, provisioning MediaWiki-Vagrant on Trusty works in that Puppet
runs to completion successfully. But the Apache setup is still broken on 2.4,
so MediaWiki isn't served.

Change-Id: I619e9ab2d61ad8fcaaafd0692e2f348493fb180a