Add an example traffic control plugin

As it is an initial implementation, it only controls latency of the
outgoing (egress) traffic. There is also a TODO to turn this plugin
into something more serious. Also, at some point we may consider
moving this plugin outside of "example" directory.

examples/plugins/traffic-control/Dockerfile

@@ -0,0 +1,7 @@
+FROM alpine:3.3
+MAINTAINER Weaveworks Inc <help@weave.works>
+LABEL works.weave.role=system
+COPY ./traffic-control /usr/bin/traffic-control
+RUN mkdir /lib64 && ln -s /lib/libc.musl-x86_64.so.1 /lib64/ld-linux-x86-64.so.2
+RUN apk add --update iproute2 && rm -rf /var/cache/apk/*
+ENTRYPOINT ["/usr/bin/traffic-control"]