Embed a copy of weaveutil instead of calling via docker

[bboreham]

Scope calls the weave script, which checks to see if it can exec weaveutil directly and, if not, execs it via a Docker image weaveexec. This PR downloads that image and copies the binary into the Scope image, which saves all the indirection and overhead of running Docker every time.

This mechanism is used when weave ps is run, which is every few seconds.

[2opremio]

Paves the way for #2276

[2opremio]

Also, should solve the problem of dead weavexec containers piling up, from a user in #weave-users: https://weaveworks.slack.com/archives/weave-users/p1487283019002156

Hey guys. I'm seeing some odd behavior with weavescope on Kubernetes. At one point in my cluster, there were over 4000 dead weaveexec containers, and it was growing. It seemed that something was launching weaveexec on the masters about every minute. After poking around a bit, I killed the weave-scope-agent DaemonSet, and that noticed that the weaveexec count stopped. To confirm, I recreated the DaemonSet, and the count started increasing again. Is this normal behavior? What exactly does weaveexec do?