test: update WeaveNet installation.

The installation of WeaveNet stopped working end of September when
cloud.weave.net was turned off. The latest upstream
documentation (https://www.weave.works/docs/net/latest/kubernetes/kube-addon/#-installation)
describes installation from GitHub.

test/setup-kubernetes.sh

@@ -276,7 +276,7 @@ kubectl version
 
 ${TEST_CONFIGURE_POST_MASTER}
 
-kubectl apply -f "https://cloud.weave.works/k8s/net?k8s-version=$(kubectl version | base64 | tr -d '\n')"
+kubectl apply -f https://github.com/weaveworks/weave/releases/download/v2.8.1/weave-daemonset-k8s.yaml
 
 # Install addon storage CRDs, needed if certain feature gates are enabled.
 # Only applicable to Kubernetes 1.13 and older. 1.14 will have them as builtin APIs.