Introduce a log.Debug constant

It's slightly more obvious that logger.V(log.Debug).Info() results in debug
logging than logger.V(1).Info().

Signed-off-by: Nic Cope <negz@rk0n.org>

pkg/log/log.go

@@ -23,6 +23,11 @@ import (
 	runtimelog "sigs.k8s.io/controller-runtime/pkg/runtime/log"
 )
 
+// Logging levels.
+const (
+	Debug = 1
+)
+
 var (
 	log = runtimelog.NewDelegatingLogger(runtimelog.NullLogger{})