Add default network setting for vnic_type to be used on network port

Currently, when neutron port is created, vnic type 'normal' is assumed in case vnic_type is not specified explicitly. We would like to add an option to define default value for vnic_type on network level , and let the default value be set on port implicitly. While the immediate need is for vnic_type implicit port definition, the mechanism to associate default setting with network can be generic enough to accommodate different attributes.