Automatic merge from submit-queue (batch tested with PRs 46076, 43879, 44897, 46556, 46654)

kubelet/network: report but tolerate errors returned from GetNetNS()

Runtimes should never return "" and nil errors, since network plugin
drivers need to treat netns differently in different cases.  So return
errors when we can't get the netns, and fix up the plugins to do the
right thing.

Namely, we don't need a NetNS on pod network teardown.  We do need
a netns for pod Status checks and for network setup.

@kubernetes/rh-networking @kubernetes/sig-network-bugs @DirectXMan12