Support version list API

The Congress API should be able to list current supported API versions, like this:

curl -g -i -X GET http://10.250.10.29:1789/

{
 "versions": [{
  "status": "CURRENT",
  "id": "v1",
  "links": [{
   "href": "http://10.250.10.29:1789/v1/",
   "rel": "self"
  }]
 }]
}