Added the ability for the user to set the notification driver


This PR adds support for the notification driver being set to either "no_op", "rpc", or "log".
Relates to : https://github.com/rcbops/chef-cookbooks/issues/651

Two new attributes are added in:
  +default["nova"]["notification"]["driver"] = "no_op"
  +default["nova"]["notification"]["topics"] = "notifications"

The driver is string matched on the driver type.