Nova server console url attribute support

Propose to add an attribute named console_url and a property named console_type to OS::Nova::Server resource.

So we can list vnc console url in outputs:

heat_template_version: 2013-05-23

resources:
  server:
    type: "OS::Nova::Server"
    properties:
      image: fedora
      key_name: heat_key
      flavor: m1.small
outputs:
  server_novnc_url:
    value:
      get_attr: [server, console_url]

Property console_type is used to specify the type of the console, now it accepts novnc, xvpvnc, spice-html5 and rdp-html5.