boot.sh: Correct use of "Alternate Value" expansion

Using `${var:+alt}` expands to the alternative value for variables which set
_and_ non-empty, while `${var+alt}` expands to the alternative value for
variables which are set, even if they are empty.

    $ ( foo="abc" ; bar="" ; echo "FOO: \"${foo+set}\" \"${foo:+set}\""; echo "BAR: \"${bar+set}\" \"${bar:+set}\""; echo "BAZ: \"${baz+set}\" \"${baz:+set}\"" )
    FOO: "set" "set"
    BAR: "set" ""
    BAZ: "" ""

The `${var:+alt}` semantics is what is needed here, it only really matters if
the variable (`${kubeadm_data}` in this case) is ever set to `""`, which it
never is here, but correct it anyway to avoid a potential future pitfall.

Signed-off-by: Ian Campbell <ijc@docker.com>

Author: Ian Campbell

boot.sh

@@ -28,10 +28,10 @@ if [ $# -eq 0 ] ; then
     # then we configure for auto init. If it is completely unset then
     # we do not.
     if [ -n "${KUBE_MASTER_AUTOINIT+x}" ] ; then
-	kubeadm_data="${kubeadm_data+$kubeadm_data, }\"init\": { \"content\": \"${KUBE_MASTER_AUTOINIT}\" }"
+	kubeadm_data="${kubeadm_data:+$kubeadm_data, }\"init\": { \"content\": \"${KUBE_MASTER_AUTOINIT}\" }"
     fi
     if [ "${KUBE_MASTER_UNTAINT}" = "y" ] ; then
-	kubeadm_data="${kubeadm_data+$kubeadm_data, }\"untaint-master\": { \"content\": \"\" }"
+	kubeadm_data="${kubeadm_data:+$kubeadm_data, }\"untaint-master\": { \"content\": \"\" }"
     fi
 
     state="kube-master-state"