feat(Makefile): conditionally set required env vars

Author: arschles

_tests/Makefile

@@ -1,3 +1,7 @@
+# These environment variables tell the tests where the deis router is, which it uses to communicate with the controller.
+# Router's default is to server on 'example.com', so 'deis.example.com' should route to controller.
+DEIS_WORKFLOW_SERVICE_HOST ?= deis.example.com
+DEIS_WORKFLOW_SERVICE_PORT ?= 80
 
 test-setup:
 	go get github.com/onsi/ginkgo/ginkgo