ensure vxlan mac will be correct with systemd-network MACAddressPolicy=random

[zhangguanzhang]

Fixes: #1795

Description

after exit, flanneld will update vtep info:

$ docker logs ae5e1a274eae
E0821 11:40:16.145538       1 main.go:335] Error registering network: failed to configure interface flannel.1: flannel.1's mac address wanto be: 36:49:83:64:e0:11 but got: 7e:fa:35:d0:2a:d0
I0821 11:40:16.145771       1 main.go:523] Stopping shutdownHandler...
$ docker ps -a | grep flanneld
b89f5d8163e2   f18629ca5ecb                                         "/opt/bin/flanneld -…"   2 seconds ago    Up 1 second                             k8s_kube-flannel_kube-flannel-ds-9h5tc_kube-system_92426357-db8b-4f70-b536-f9de959d0881_7
ae5e1a274eae   f18629ca5ecb                                         "/opt/bin/flanneld -…"   23 seconds ago   Exited (1) 14 seconds ago               k8s_kube-flannel_kube-flannel-ds-9h5tc_kube-system_92426357-db8b-4f70-b536-f9de959d0881_6

Todos

• Tests
• Documentation
• Release note

Release Note

None required

[zhangguanzhang]

@rbrtbnfgl PTAL