Add --feature-gates to kube-system components

apiserver,scheduler,controller-manager,proxy,kubelet all get
flag. Using one variable to plumb through config via salt/init
scripts for GCE and GKE

cluster/common.sh

@@ -682,6 +682,11 @@ EOF
     if [ -n "${INITIAL_ETCD_CLUSTER:-}" ]; then
       cat >>$file <<EOF
 INITIAL_ETCD_CLUSTER: $(yaml-quote ${INITIAL_ETCD_CLUSTER})
+EOF
+    fi
+    if [ -n "${FEATURE_GATES:-}" ]; then
+      cat >>$file <<EOF
+FEATURE_GATES: $(yaml-quote ${FEATURE_GATES})
 EOF
     fi
   else