Change hardcoded nightly repo to variable

Change-Id: Ifbe7369ddc2937f8abd406eb7097a67d5373c863
diff --git a/aptly/server/mirror/ubuntu/trusty/salt.yml b/aptly/server/mirror/ubuntu/trusty/salt.yml
index bfc4168..4c243b8 100644
--- a/aptly/server/mirror/ubuntu/trusty/salt.yml
+++ b/aptly/server/mirror/ubuntu/trusty/salt.yml
@@ -1,4 +1,6 @@
 parameters:
+  _param:
+    apt_mk_version: stable
   aptly:
     server:
       mirror:
@@ -12,5 +14,5 @@
           publisher:
             component: salt
             distributions:
-              - ubuntu-trusty/nightly
+              - ubuntu-trusty/${_param:apt_mk_version}