Automatic merge from submit-queue

network/cni: Unconditionally bring up `lo` interface

This is already done in kubenet. This specifically fixes an issue where a kubelet-managed network for the rkt runtime does not have an "UP" lo interface.

Fixes #28561

If this fix doesn't seem right, it could also be implemented by rkt effectively managing two "cni" network plugins, one for the user requested network, one for lo.

Followup CRs can improve unit testing further and then possibly remove the vendor directory logic (which seems like dead code)

cc @kubernetes/sig-rktnetes @kubernetes/sig-network @dcbw