Sample debugging configuration for MediaWiki

This patch adds '00-debug.php-example' to 'settings.d/' to fulfill two goals:
first, it serves as an illustration of how to use 'settings.d/' for
managing configuration snippets. Second, it makes it easy to actually enable
useful debug settings for MediaWiki development.

This patch also configures Puppet to deploy 'php-development-settings.ini' to
PHP's 'conf.d' directory. This ini file configures PHP for development work by
enabling verbose error output.

Change-Id: I1340609deafec14d6c2a522e6ff4e08a89f2811f