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/travis-kube-setup.sh

@@ -6,6 +6,14 @@
 
 set -e 
 
+# If we don't have gcloud credentials, bail out of these tests.
+if [ -z ${FISSION_CI_SERVICE_ACCOUNT:+} ]
+then
+    echo "Skipping tests, no cluster credentials"
+    exit 0
+fi
+
+# Get kubectl
 curl -LO https://storage.googleapis.com/kubernetes-release/release/$(curl -s https://storage.googleapis.com/kubernetes-release/release/stable.txt)/bin/linux/amd64/kubectl
 chmod +x ./kubectl
 sudo mv ./kubectl /usr/local/bin/kubectl