[API] add more details for valence in api root

[Lin Yang]

Current valence description is too simple in api root as below shows, should add more details.

$curl -vvvvvv -X GET http://127.0.0.1:8181/

{
  "description": "Valence is an OpenStack project",
  "name": "OpenStack Valence API",
  "versions": [
    {
      "id": "v1",
      "links": [
        {
          "href": "http://127.0.0.1:8181/v1/",
          "rel": "self"
        }
      ],
      "min_version": "",
      "status": "CURRENT"
    }
  ]
}