Add console_urls attribute to server resource

This is a composite attribute that returns a dict-like structure
lazily resolving the actual console URL based on key supplied or dict of
all available consoles if no key is provided.
Example of usage to get only 'novnc' console:

  outputs:
    console:
      value: { get_attr: [server, console_urls, novnc ] }

Change-Id: I27c0426f1a9100bb483f04c059fec87bb69b6340
Implements: blueprint vnc-console-attr
Co-Authored-By: Jun Jie Nan <nanjj@cn.ibm.com>
Merge "Add console_urls attribute to server resource"