base_url modification

Allow modification of novncproxy_base_url and xvpvncproxy_base_url using variables.

E.G
  "override_attributes": {
    "nova": {
      "novnc": {
        "base_url": "http://8.8.8.8:6080/vnc_auto.html"
      },
      "xvpvnc": {
        "base_url": "http://8.8.8.8:6081/console"
      },
}