Manage Include directives without managing the file.

This adds support for the new value 'onlyinclude' for the $ensure
parameter of apache::include_file.  Setting 'onlyinclude' will add
an Include directive to the main httpd.conf file without installing
(or otherwise managing) the file that is included.

This is useful if the included file is installed in some other way,
e.g. via package (RPM/Deb/ebuild/...) installation, or by using the
concat::file definition.  The latter is the direct motivation for
this feature; we will use that to manage Listen directives in the
upcoming commit.