Use hostUrl, not apiUrl

Asking for apiUrl in config is misleading, as /api/... is appended to most calls anyway. Instead, we'll use hostUrl.

{
  "configuration": {
    "username": "admin",
    "password": "district",
    "hostUrl": "https://play.dhis2.org/2.31.1"
  },
  "data": {
    "a": 1,
    "b": 2,
  }
}