Network default settings extension

There are different setting that are applicable on neutron port level, especially with various port extension. The flow of VM provisioning requires port creation and then 'nova boot' using created port_id. There are cases when there are common port settings applicable to majority of network ports. Applying common port setting on network level and inheriting it during port creation will allow VM provisioning to leverage 'nova boot' command using net_id, without port being pre-created. During the port creation, it should be possible to override default settings. New extension will be introduced in order to define network default settings. First candidate to consume this will be ML2 plugin with regards to vnic_type setting.