spec: remove the version parameter from ADD and DEL commands.
spec: remove the version parameter from ADD and DEL commands.

The CNI spec version passed from runtime to plugin is the `cniVersion`
field in the network configuration rather than a parameter to the ADD
and DEL command. So, remove the version parameter from the spec, and
also describe how the `cniVersion` field in the network configuration
should be used by the plugin.

Fixes #542

containernetworking/cni,appc/cni,dcbw/cni,dcbw/cni,containernetworking/cni,appc/cni