This change is to add CNI_ARGS support in static IPAM plugin.
When IP/SUBNET/GATEWAY are given in CNI_ARGS, static IPAM adds
these info in addition to config files.
To configure ip address only from CNI_ARGS, 'address' field in config
is changed to optional from required.
Recent CNI specification changes require the container ID on ADD/DEL,
which the testcases were not providing. Fix that up so things work
when this repo gets CNI revendored.