Addon Manager: Fix parsing KUBECTL_EXTRA_PRUNE_WHITELIST

When there are two or more values in KUBECTL_EXTRA_PRUNE_WHITELIST
kube-addons.sh wrongly generates "--prune-whitelist value1 value2"
Which kubectl parses as additional unknown flag[s]

Change-Id: I7661bda41ab7435e0e65eb0a662d1b16d2c14991

cluster/addons/addon-manager/kube-addons.sh

@@ -120,7 +120,7 @@ function generate_prune_whitelist_flags() {
 # besides the default ones.
 extra_prune_whitelist=
 if [ -n "${KUBECTL_EXTRA_PRUNE_WHITELIST:-}" ]; then
-  extra_prune_whitelist=( "${KUBECTL_EXTRA_PRUNE_WHITELIST:-}" )
+  read -ra extra_prune_whitelist <<< "${KUBECTL_EXTRA_PRUNE_WHITELIST}"
 fi
 prune_whitelist=( "${KUBECTL_PRUNE_WHITELIST[@]}"  "${extra_prune_whitelist[@]}" )
 prune_whitelist_flags=$(generate_prune_whitelist_flags "${prune_whitelist[@]}")