stop weavedns from complaining when a container has no docker ip
e.g. when it was started with --net=none

weavedns wasn't actually using the docker-assigned ip for anything, so
this change is a simple matter of eradicating it from the code.
s/weaveIP/ip
Merge pull request #243 from rade/240_dns_without_docker_net

LGTM.  Fixes #240