proxy needs to add weavewait to execs as well

[paulbellamy]

Bug came up in a test where we are restarting and doing an exec checking
a network interface right away. weavewait is only added if the container
has a WEAVE_CIDR var set, the same as at container creation.

[paulbellamy]

re: state UNKNOWN.
weave/net.EnsureInterface, does check the that the interface has the Up flag set, so it should definitely be waiting. I believe it's ip link show ethwe being a bit overly-specific/racey. ifconfig ethwe shows the interface as UP.

[rade]

That seems a little odd. Could it be timing related, e.g. perhaps ifconfig is slower than ip? Or perhaps ip works off some information that is updated asynchronously. I wonder whether our resident kernel sleuth knows the answer. @dpw?