[proxy] use logs for getting proxy-env and proxy-config

It's ugly, but beats arg-parsing, inspecting the container and piecing
the address together from exposed ports.

We could just use the host from DOCKER_HOST, but that will fail in the
case of a host with multiple IPs, where the proxy only listens on one.

test/650_proxy_env_test.sh

@@ -0,0 +1,13 @@
+#! /bin/bash
+
+. ./config.sh
+
+start_suite "Configure the docker daemon for the proxy"
+
+weave_on $HOST1 launch-proxy
+
+CMD="run -e WEAVE_CIDR=10.2.1.4/24 $BASE_IMAGE $CHECK_ETHWE_UP"
+assert_raises "eval '$(weave_on $HOST1 proxy-env)' ; docker $CMD"
+assert_raises "docker $(weave_on $HOST1 proxy-config) $CMD"
+
+end_suite