Fix doc generation for Rackspace::Cloud::Network

Since contrib plugins may not be installed at the time the docs are
generated, the resource_mapping function needs to return all resource
types defined by the resource module.  To selectively load such resource
types - based on requirements being met, for instance, the
available_resource_mapping function should be used instead.

This makes the necessary changes to Rackspace::Cloud::Network so that
its docs are generated correctly.

Related-Bug: 1403897
Change-Id: I7c12c055924e7a16a5b9cc573d91c12089a2395d