apache: add env-{available,enabled}

Extend the Debian Apache configuration directory pattern by adding a fourth
type: /etc/apache2/env-{available,enabled}. We've been doing this in
MediaWiki-Vagrant for a while and it works well because it allows you to clean
up after roles much more easily than could be done by directly modifying
envvars. With this approach, only one modification to the stock envvars file is
necessary: the addition for a line that recurses into env-enabled.

Change-Id: Ib80d211afdda3b3a1c41f4e7f2459137153d2b39