make DNS work when docker is running with --icc=false
1) expose the DNS port from the weavedns container
2) link all application containers to the weavedns container
Merge pull request #173 from rade/141_dns_without_icc

LGTM.  Closes #141
expose DNS port via Dockerfile rather than 'docker run' argument
That is "the Docker way".

Related to #141.