webserver::apache: misc adjustments

- Change default log location for virtualhosts into
  /var/log/$name.{access,error}.log. This does the right thing on
  servers with multiple sites.
- AllowOverride All on the main docroot. Since the config for that is
  static, being able to use .htaccess can prove handy, and is in fact
  what LibreNMS does.

Change-Id: I17e66c874194457675a7a5347961c953b867c4bc