plugins: add plugins from containernetworking/cni

plugins: add plugins from containernetworking/cni

Plugins prepared from the containernetworking/cni repo as follows:

1) git reset --hard 1a9288c3c09cea4e580fdb1a636f1c5e185a391f
2) git remove everything not in plugins/
3) git remove plugins/test
4) git merge into containernetworking/plugins repo
5) adjust import paths for containernetworking/cni -> containernetworking/plugins