Make maas saltstack_repo's logic more general

Change-Id: I71021489b79c27d9b46b796c34ded21c4ee6d734
diff --git a/metadata/service/cluster/single.yml b/metadata/service/cluster/single.yml
index a307dcc..de8cf8a 100644
--- a/metadata/service/cluster/single.yml
+++ b/metadata/service/cluster/single.yml
@@ -41,5 +41,5 @@
         MA==
         =dtMN
         -----END PGP PUBLIC KEY BLOCK-----
-      saltstack_repo_trusty: "http://repo.saltstack.com/apt/ubuntu/14.04/amd64/2016.3/ trusty main"
-      saltstack_repo_xenial: "http://repo.saltstack.com/apt/ubuntu/16.04/amd64/2016.3/ xenial main"
+      saltstack_repo_trusty: "deb [arch=amd64] http://repo.saltstack.com/apt/ubuntu/14.04/amd64/2016.3/ trusty main"
+      saltstack_repo_xenial: "deb [arch=amd64] http://repo.saltstack.com/apt/ubuntu/16.04/amd64/2016.3/ xenial main"