Add plugin code

This adds basic plugins.
"main" types: veth, bridge, macvlan
"ipam" type: host-local

The code has been ported over from github.com/coreos/rkt project
and adapted to fit the CNI spec.

scripts/priv-net-run.sh

@@ -0,0 +1,20 @@
+#!/bin/bash -e
+
+# Run a command in a private network namespace
+# set up by CNI plugins
+
+netnsname=$(printf '%x%x' $RANDOM $RANDOM)
+netnspath=/var/run/netns/$netnsname
+
+ip netns add $netnsname
+ip netns exec $netnsname ip link set lo up
+./exec-plugins.sh add $netnspath
+
+
+function cleanup() {
+	./exec-plugins.sh del $netnspath
+	ip netns delete $netnsname
+}
+trap cleanup EXIT
+
+ip netns exec $netnsname $@