restarting app containers with network re-attachment

[rade]

docker restart restarts containers. If one runs that on a weave application container, then the new incarnation won't be attached to the weave network. Which is less than ideal.

Fixing this properly requires resolution of #230/#312. Meanwhile we could do what we've done for other docker commands: have an equivalent weave command, i.e. weave restart, which would extract the existing weave network settings from the containers interface, run docker restart and then the equivalent of weave attach. Possibly something might also have to be done about (re)registering with weavedns.

[rade]

NB: this issue only covers docker restart, not restarting of containers when the docker daemon restarts, which is the subject of #401. And it also only covers the case where the CIDR is supplied at container creation time, not afterwards, with weave attach, which is the subject of #688.