namespaces: Export env var and default value

Signed-off-by: Samuel Karp <skarp@amazon.com>

namespaces/context_test.go

@@ -31,8 +31,8 @@ func TestContext(t *testing.T) {
 }
 
 func TestNamespaceFromEnv(t *testing.T) {
-	oldenv := os.Getenv(namespaceEnvVar)
-	defer os.Setenv(namespaceEnvVar, oldenv) // restore old env var
+	oldenv := os.Getenv(NamespaceEnvVar)
+	defer os.Setenv(NamespaceEnvVar, oldenv) // restore old env var
 
 	ctx := context.Background()
 	namespace, ok := Namespace(ctx)
@@ -45,7 +45,7 @@ func TestNamespaceFromEnv(t *testing.T) {
 	}
 
 	expected := "test-namespace"
-	os.Setenv(namespaceEnvVar, expected)
+	os.Setenv(NamespaceEnvVar, expected)
 	nctx := NamespaceFromEnv(ctx)
 
 	namespace, ok = Namespace(nctx)