CNI_NETNS should not be set for orphan containers that do not have network namespace.

We introduced a new agent flag in MESOS-9492 so that CNI configs can be persisted across reboot. This is for some CNI plugins to be able to cleanup IP allocated to the containers after a sudden reboot of the host (not all CNI plugins need this).

It's important to unset `CNI_NETNS` environment variable after reboot when invoking CNI plugin "DEL" command so that it conforms to the spec:

When CNI_NETNS and/or prevResult are not provided, the plugin should clean up as many resources as possible (e.g. releasing IPAM allocations) and return a successful response.