pyang: add missing key for list statement

pyang: add missing key for list statement

This patch fixes following pyang errors

> neutron.yang:38: error: the list at "neutron.yang:38 (at neutron-L3.yang:155)" needs at least one key because it is used as config
> neutron.yang:40: error: the list at "neutron.yang:40 (at neutron-metering.yang:80)" needs at least one key because it is used as config
> neutron.yang:41: error: the list at "neutron.yang:41 (at neutron-metering.yang:90)" needs at least one key because it is used as config
> neutron.yang:42: error: the list at "neutron.yang:42 (at neutron-ports.yang:33)" needs at least one key because it is used as config

Change-Id: Iebfd6a0233a37eac539613c7f3e6cf765f9a0cee
Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>