Change kubemark Makefile to be provider independent

The Kubemark Makefile is defaulted to gcr.io. Instead, make it
provider independent.

cluster/images/kubemark/Makefile

@@ -15,8 +15,18 @@
 # build Kubemark image from currently built binaries containing both 'real' master and Hollow Node.
 # This makefile assumes that the kubemark binary is present in this directory.
 
-all:
-	docker build --pull -t gcr.io/$(PROJECT)/kubemark .
-	gcloud docker -- push gcr.io/$(PROJECT)/kubemark
+REGISTRY?=gcr.io
+PROJECT?=google_containers
 
-.PHONY: all
+all: gcloudpush
+
+build:
+	docker build --pull -t $(REGISTRY)/$(PROJECT)/kubemark .
+
+gcloudpush: build
+	gcloud docker -- push $(REGISTRY)/$(PROJECT)/kubemark
+
+push: build
+	docker -- push $(REGISTRY)/$(PROJECT)/kubemark
+
+.PHONY: all build gcloudpush push