Update flannel version to 0.15.1

story: 2007256
task: 44417
Change-Id: I383bdd9e5362a252746dbe185037a888db6dc85a

Author: GregWhiteyBialas

magnum/drivers/k8s_fedora_coreos_v1/templates/kubecluster.yaml

@@ -500,7 +500,7 @@ parameters:
   flannel_tag:
     type: string
     description: tag of the flannel container
-    default: v0.12.0-amd64
+    default: v0.15.1
 
   flannel_cni_tag:
     type: string

releasenotes/notes/update-flannel-version.yaml

@@ -0,0 +1,8 @@
+---
+upgrades:
+  - |
+    Updates flannel to version 0.15.1.
+    This will address issue where pods on multinode
+    installations don't have network connectivity
+    if they are spawned on different hosts.
+    `More details <https://github.com/flannel-io/flannel/blob/master/Documentation/troubleshooting.md>`_