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.
Namespace creation had an unergonomic interface and isn't used, except
for testing code. Remove it; downstream users should really be creating
their own namespaces
On ADD save the host device's name into its IFLA_ALIAS property and
rename the device to the requested CNI_IFNAME inside the container
to conform to the CNI specification. On DEL rename the device to
the original name and move it back into the host namespace.
