Add URL to HREF field for available plugin or remove HREF field

[geauxvirtual]

When returning available plugins from the REST API, the available plugin has an empty HREF field. The field should either be populated with the HREF for the plugin or be removed from the return from the REST API.

"available_plugins": [
      {
        "name": "passthru",
        "version": 1,
        "type": "processor",
        "hitcount": 19,
        "last_hit_timestamp": 1460046641,
        "id": 1,
        "href": ""
      },