with ifupdown2, we can't call ifup for each ovs_ports in OvsBridge.
But as we define also ovs_bridge for each interface, we can simply add
the bridge here, to be sure that it's exist before adding the interface.
for config, we simply need to replace allow-..., with auto.
I have keeped ovs_ports to avoid config change, but it's not used.
old config
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auto vmbr100
iface vmbr100 inet manual
ovs_type OVSBridge
ovs_ports bond0 ovsinternalport