use plugin registry and prepare for conversion (#1003)

* use plugin registry and prepare for conersion

* Register plugins explicitly to a registry

* check interface impl instead of struc var

* setup plugins at top level

* treat plugin type combinations

* pass registry as arg of V1alpha1ToInternal

* move registry yet another level up

* check interface type separately

test/e2e/e2e_leaderelection_test.go

@@ -38,6 +38,7 @@ import (
 )
 
 func TestLeaderElection(t *testing.T) {
+	descheduler.SetupPlugins()
 	ctx := context.Background()
 
 	clientSet, _, _, _, stopCh := initializeClient(t)