Flannel is recommending using vxlan[1] and udp is just for debugging
or the kernel doesn't support vxlan or host-gw. So this patch is
proposing using 'vxlan' as the default value of label 'flannel_backend'
and it has been verified with sonobuoy.
[1] https://github.com/coreos/flannel/blob/master/Documentation/backends.md
Task: 36425
Story: 2006482
Change-Id: Ibe7f3446be894c593c6147186cc159bd01834d29