Allow override of discovery URL via ConfigMap

Signed-off-by: Andrew Keesler <akeesler@vmware.com>

- Seems like the next step is to allow override of the CA bundle; I didn't
  do that here for simplicity of the commit, but seems like it is the right
  thing to do in the future.

test/library/env.go

@@ -0,0 +1,22 @@
+/*
+Copyright 2020 VMware, Inc.
+SPDX-License-Identifier: Apache-2.0
+*/
+
+package library
+
+import (
+	"os"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+// Getenv gets the environment variable with key and asserts that it is not
+// empty. It returns the value of the environment variable.
+func Getenv(t *testing.T, key string) string {
+	t.Helper()
+	value := os.Getenv(key)
+	require.NotEmptyf(t, value, "must specify %s env var for integration tests", key)
+	return value
+}