Add a default policy file, install it in (make install) and integration tests

(skopeo copy) will soon ALWAYS require a present policy file.  So,
install one by (make install), and ensure that integration tests do so
as well.

Also simplifies the usage of install(1) a bit.

hack/make/test-integration

@@ -9,7 +9,7 @@ bundle_test_integration() {
 # subshell so that we can export PATH without breaking other things
 (
 	make binary-local
-	make install-binary
+	make install
 	export GO15VENDOREXPERIMENT=1
 	bundle_test_integration
 ) 2>&1