As of OSP15, the default ML2 mechanism driver is the OVN mechanism driver. (In OSP14 the default was the ML2/OVN mechanism driver.) This change is documented in the OSP15 Release Notes Top New Features chatper. In addition we need to add statements in the guides, including the Networking Guide and the OVN Guide. We may need statements in other guides as well. We also need to update configuration file and command descriptions as needed.