Update implementation and remove TODO comment

* openstack_dashboard/api/nova.py:
  (server_update): Update implementation since novaclient.servers.update() has
   changed.

   See (python-novaclient/in63073104665ee4597cf3b7aa8dc2295a8a7db794)

Closes-Bug: #1208714

Change-Id: I80e9b1c1998a32df4229e25fbd3e089bfbf609f0
Merge "Update implementation and remove TODO comment"