Add --log-level support

Next steps, in another PR, would be:
- make all logging go through the logrus stuff
- I'd like to see if we can remove the env var stuff (like DEBUG) but we'll see

Closes #5198

Signed-off-by: Doug Davis <dug@us.ibm.com>

docker/log.go

@@ -6,11 +6,7 @@ import (
 	log "github.com/Sirupsen/logrus"
 )
 
-func initLogging(debug bool) {
+func initLogging(lvl log.Level) {
 	log.SetOutput(os.Stderr)
-	if debug {
-		log.SetLevel(log.DebugLevel)
-	} else {
-		log.SetLevel(log.InfoLevel)
-	}
+	log.SetLevel(lvl)
 }