Document changes to the overridden methods of the nova driver

Change-Id: I02ecfca0a5b3b9ba9437753cae8dc45f69820cf1
Signed-off-by: Rosario Di Somma <rosario.disomma@dreamhost.com>
Merge pull request #23 from rodis/stable/icehouse

Document changes to the overridden methods of the nova driver