yang model revise: neutron-fwaas.yang

yang model revise: neutron-fwaas.yang

This patch revises yang model of neutron fwaas and related codes.
- fix tcp/udp port
- introduce identity and identityref for protocol/action
- drop descr for consistency
- drop backpointers. firewall-rules
  If openstack provider wants a list of rules associated with
  a given firewall policy, walk through rules and find rules whose
  policy is the uuid of the policy.
- drop status in firewall policy which isn't defined in neutron API
- eliminated deprecated methods in neutron-spi

Change-Id: Ie6fac4202fadcff718d0d4960230eefe06852bc6
Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
Signed-off-by: Ritu Sood <ritu.sood@intel.com>