Use unique socket filename when registering with the kubelet

[cdesiniotis]

kubernetes/kubernetes#135437 introduces the ability to rollout device plugins without downtime (e.g. maxSurge > 0). In order to support this, plugins must use unique socket filenames when registering with the kubelet. From https://github.com/kubernetes/kubernetes/blob/master/pkg/kubelet/pluginmanager/pluginwatcher/README.md#seamless-upgrade:

To avoid downtime of a plugin on a node, it would be nice to support running an old plugin in parallel to the new plugin. When deploying with a DaemonSet, setting maxSurge to a value larger than zero enables such a seamless upgrade.

To support seamless upgrades, each plugin instance must use a unique socket filename.