New definition for managing included config files.

This adds apache::include_file, a definition for managing "include
files", put into /etc/httpd/conf.d and loaded using Include directives
from the main httpd.conf configuration file.

This can be used for e.g. managing virtual hosts and/or global
configuration for Apache modules (like adding LoadModule directives).
(It is expected that higher level defines will be added for those
kind of things, so end-users don't need to use apache::include_file
directly themselves.)