PLEG is reponsible for listing the pods running on the node. If it's hung
due to non-responsive container runtime or internal bugs, we should restart
kubelet.
This change introduces pod lifecycle event generator (PLEG), and adds a generic
PLEG. The generic PLEG relies on relisting to discover container events, and is
container-runtime-agnostic. Both docker and rkt are changed to use generic
PLEG.