Change hardcoded nightly repo to variable

Change-Id: Ifbe7369ddc2937f8abd406eb7097a67d5373c863
diff --git a/aptly/server/mirror/ubuntu/xenial/glusterfs.yml b/aptly/server/mirror/ubuntu/xenial/glusterfs.yml
index f8177a0..248de32 100644
--- a/aptly/server/mirror/ubuntu/xenial/glusterfs.yml
+++ b/aptly/server/mirror/ubuntu/xenial/glusterfs.yml
@@ -1,5 +1,6 @@
 parameters:
   _param:
+    apt_mk_version: stable
     glusterfs_version: 3.8
   aptly:
     server:
@@ -14,4 +15,4 @@
           publisher:
             component: glusterfs
             distributions:
-              - ubuntu-xenial/nightly
+              - ubuntu-xenial/${_param:apt_mk_version}