When Network Service (NS) is supported in Tacker, we can launch
multiple VNFFGs by initiating NS. Because each VNFFG can have
different 'symmetrical' value, we should support dynamic 'symmetrical'
parameter for each VNFFG in VNFFG descriptor.
This feature will add 'symmetrical' as a property of forwarding path,
so users can set 'symmetrical' value to 'true' or 'false' to change
symmetrical value of a port chain in networking-sfc.
Change-Id: Ie9a6abe96141954e95902cd426abafa10d744236
df8be52ac7