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/ubuntu/util.sh

@@ -339,7 +339,7 @@ function kube-up() {
   source "${KUBE_ROOT}/cluster/common.sh"
 
   # set kubernetes user and password
-  gen-kube-basicauth
+  load-or-gen-kube-basicauth
 
   create-kubeconfig
 }