backend: Make Makefile image variables consistent with frontend

The image variables in frontend/Makefile changed recently in
#1011

This just makes backend/Makefile use consistent variable names.

Author: Matthew Barnes

backend/Makefile

@@ -3,9 +3,9 @@
 HELM_CMD ?= helm upgrade --install
 
 CURRENT_COMMIT := $(shell git rev-parse --short=7 HEAD)
-
-ARO_HCP_BASE_IMAGE ?= ${ARO_HCP_IMAGE_ACR}.azurecr.io
-ARO_HCP_BACKEND_IMAGE ?= $(ARO_HCP_BASE_IMAGE)/arohcpbackend
+ARO_HCP_IMAGE_REGISTRY ?= ${ARO_HCP_IMAGE_ACR}.azurecr.io
+ARO_HCP_IMAGE_REPOSITORY ?= arohcpbackend
+ARO_HCP_BACKEND_IMAGE ?= $(ARO_HCP_IMAGE_REGISTRY)/$(ARO_HCP_IMAGE_REPOSITORY)
 
 .DEFAULT_GOAL := backend