AWS kube-up: Allow spot instances

I think we should probably leave this undocumented for now, until we
have a better way to launch multiple sets of nodes, but it's great for
cost savings while testing!

Fix kubernetes#21200

cluster/aws/util.sh

@@ -1121,6 +1121,12 @@ function start-minions() {
   else
     public_ip_option="--no-associate-public-ip-address"
   fi
+  local spot_price_option
+  if [[ -n "${NODE_SPOT_PRICE:-}" ]]; then
+    spot_price_option="--spot-price ${NODE_SPOT_PRICE}"
+  else
+    spot_price_option=""
+  fi
   ${AWS_ASG_CMD} create-launch-configuration \
       --launch-configuration-name ${ASG_NAME} \
       --image-id $KUBE_NODE_IMAGE \
@@ -1129,6 +1135,7 @@ function start-minions() {
       --key-name ${AWS_SSH_KEY_NAME} \
       --security-groups ${NODE_SG_ID} \
       ${public_ip_option} \
+      ${spot_price_option} \
       --block-device-mappings "${NODE_BLOCK_DEVICE_MAPPINGS}" \
       --user-data "fileb://${KUBE_TEMP}/node-user-data.gz"