Don't set KUBECONFIG when there is no reachable cluster

Travis doesn't pass secret env vars to tests run on forks, so we need
a way to skip the tests when there are no credentials passed in.

This change unsets KUBECONFIG when there's no reachable cluster. The
Go tests already check for KUBECONFIG and skip when it's not defined.

hack/runtests.sh

@@ -3,5 +3,10 @@
 # The poolmgr unit test only works with NodePort-type services for
 # now. So disable it for our travis ci tests.
 
+if [ ! -f ${KUBECONFIG} ]
+then
+    unset KUBECONFIG
+fi
+
 go test -v -i $(go list ./... | grep -v '/vendor/' | grep -v 'examples/go')
 go test -v $(go list ./... | grep -v '/vendor/' | grep -v 'examples/go' | grep -v poolmgr)