Add pod eviction logic for scheduler preemption

Add Preempt to scheduler interface
Add preemption to the scheduling workflow
Minor changes to the scheduler integration test library

plugin/pkg/scheduler/algorithm/scheduler_interface.go

@@ -47,6 +47,10 @@ type SchedulerExtender interface {
 // onto machines.
 type ScheduleAlgorithm interface {
 	Schedule(*v1.Pod, NodeLister) (selectedMachine string, err error)
+	// Preempt receives scheduling errors for a pod and tries to create room for
+	// the pod by preempting lower priority pods if possible.
+	// It returns the node where preemption happened, a list of preempted pods, and error if any.
+	Preempt(*v1.Pod, NodeLister, error) (selectedNode *v1.Node, preemptedPods []*v1.Pod, err error)
 	// Predicates() returns a pointer to a map of predicate functions. This is
 	// exposed for testing.
 	Predicates() map[string]FitPredicate