Add support for NAT output feature to

https://git.fd.io/hc2vpp/tree/nat/nat-api/src/main/yang/interface-nat@2016-12-14.yang

Provide translation layer (snat_interface_add_del_output_feature)

Renaming some nodes might be needed to better express difference between

snat_interface_add_del_output_feature  and snat_interface_add_del_feature we used so far.