Issue title: Leftover bridge after installing weave CNI kubernetes plugin
 Issue body: After we installed weave (1.6.2) on a 4-node kube cluster, the weave CNI plugin failed to clean up existing conflicting routes for the `cbr0` bridge.

Once deleting the route for `cbr0`, inter-node connectivity worked as expected.

@bboreham 
 Comments: 
 Comment 0: I looked into this, and I think the resolution is to make Kubernetes stop creating the cbr0 bridge and routes.  I have submitted https://github.com/kubernetes/kubernetes/pull/33438 to fix

 Comment 1: Also I [moved the address range back to the Weave default](https://github.com/weaveworks/weave-kube/commit/0541a072f1d9554beecbfd45e3a9a89954762cb8), so it won't clash with the Kubernetes routes
 Comment 2: Thank you, @bboreham !!!
{'source': 'GithubIssues', 'repo_name_with_owner': 'weaveworks/weave', 'issue_id': 178449585, 'repo_stars': 3798.0}