Load or generate auth from kubeconfig on kube-up

Allows loading existing auth from kubeconfig on kube-up if a
valid KUBE_CONTEXT is specified, instead of always force
regenerating auth (basic or token) when creating a new cluster.

cluster/vagrant/util.sh

@@ -297,7 +297,7 @@ function verify-cluster {
 
 # Instantiate a kubernetes cluster
 function kube-up {
-  gen-kube-basicauth
+  load-or-gen-kube-basicauth
   get-tokens
   create-provision-scripts