Parameterize config watcher loop speed

[mingshuoqiu]

The original PR is https://github.com/k8snetworkplumbingwg/multus-cni/pull/1232. This PR rebase the commit on top of the current master.

This loop spins really quickly otherwise. This will allow cluster operators to choose how quickly the thin plugin reconciles the kubeconfig in the event of a stale service account token.