[proxy] add --rewrite-inspect flag

[paulbellamy]

To return the weave ip/mac for docker inspect calls.

Blocked on a pr to go-dockerclient being merged: fsouza/go-dockerclient#336

@errordeveloper, Could you see if this works for you?

Possibly, fixes #1199?

[paulbellamy]

Closed while being tried out. Needs testing for the USR2 change, if it works.

[errordeveloper]

Yeah, looks like it works, just testing from scratch now. Will update.

[errordeveloper]

Ok, so just for the record, this appears to work, however I am seeing a strange issue which looks like only some /w/w processes get SIGUSR2. I am not quite sure, but due to the signal having to go via Docker and my overcomited VMs might struggle and hit some edge case where Docker does't end-up sending those signals. To confirm, I will have to trace whether Docker calls signal() or what not, as well as read the code. According to the logs, it's supposed to, but I am not entirely convinced with that being a guarantee. I've checked with David, and he say that signals cannot just get dropped, although they may get blocked.