AWS kube-up: Clean up distro handling

The only tested-working distros are vivid, wily & jessie.

vivid should not really be used because it is no longer supported, so
recommend wily or jessie instead.

For other distros, recommend jessie instead.

Fix kubernetes#21218

cluster/aws/util.sh

@@ -49,24 +49,51 @@ MASTER_DISK_ID=
 # Well known tags
 TAG_KEY_MASTER_IP="kubernetes.io/master-ip"
 
+OS_DISTRIBUTION=${KUBE_OS_DISTRIBUTION}
+
 # Defaults: ubuntu -> wily
-if [[ "${KUBE_OS_DISTRIBUTION}" == "ubuntu" ]]; then
-  KUBE_OS_DISTRIBUTION=wily
+if [[ "${OS_DISTRIBUTION}" == "ubuntu" ]]; then
+  OS_DISTRIBUTION=wily
 fi
 
-# For GCE script compatibility
-OS_DISTRIBUTION=${KUBE_OS_DISTRIBUTION}
-
-case "${KUBE_OS_DISTRIBUTION}" in
-  trusty|wheezy|jessie|vivid|wily|coreos)
-    source "${KUBE_ROOT}/cluster/aws/${KUBE_OS_DISTRIBUTION}/util.sh"
+# Loads the distro-specific utils script.
+# If the distro is not recommended, prints warnings or exits.
+function load_distro_utils () {
+case "${OS_DISTRIBUTION}" in
+  jessie)
+    ;;
+  wily)
+    ;;
+  vivid)
+    echo "vivid is currently end-of-life and does not get updates." >&2
+    echo "Please consider using wily or jessie instead" >&2
+    echo "(will continue in 10 seconds)" >&2
+    sleep 10
+    ;;
+  coreos)
+    echo "coreos is no longer supported by kube-up; please use jessie instead" >&2
+    exit 2
+    ;;
+  trusty)
+    echo "trusty is no longer supported by kube-up; please use jessie or wily instead" >&2
+    exit 2
+    ;;
+  wheezy)
+    echo "wheezy is no longer supported by kube-up; please use jessie instead" >&2
+    exit 2
     ;;
   *)
-    echo "Cannot start cluster using os distro: ${KUBE_OS_DISTRIBUTION}" >&2
+    echo "Cannot start cluster using os distro: ${OS_DISTRIBUTION}" >&2
+    echo "The current recommended distro is jessie" >&2
     exit 2
     ;;
 esac
 
+source "${KUBE_ROOT}/cluster/aws/${OS_DISTRIBUTION}/util.sh"
+}
+
+load_distro_utils
+
 # This removes the final character in bash (somehow)
 AWS_REGION=${ZONE%?}
 
@@ -266,7 +293,7 @@ function detect-security-groups {
 # Vars set:
 #   AWS_IMAGE
 function detect-image () {
-case "${KUBE_OS_DISTRIBUTION}" in
+case "${OS_DISTRIBUTION}" in
   trusty|coreos)
     detect-trusty-image
     ;;
@@ -283,7 +310,7 @@ case "${KUBE_OS_DISTRIBUTION}" in
     detect-jessie-image
     ;;
   *)
-    echo "Please specify AWS_IMAGE directly (distro ${KUBE_OS_DISTRIBUTION} not recognized)"
+    echo "Please specify AWS_IMAGE directly (distro ${OS_DISTRIBUTION} not recognized)"
     exit 2
     ;;
 esac
@@ -834,7 +861,7 @@ function subnet-setup {
 }
 
 function kube-up {
-  echo "Starting cluster using os distro: ${KUBE_OS_DISTRIBUTION}" >&2
+  echo "Starting cluster using os distro: ${OS_DISTRIBUTION}" >&2
 
   get-tokens