Change hardcoded nightly repo to variable

Change-Id: Ifbe7369ddc2937f8abd406eb7097a67d5373c863
diff --git a/aptly/server/mirror/debian/cassandra.yml b/aptly/server/mirror/debian/cassandra.yml
index 3d32e6a..846df4e 100644
--- a/aptly/server/mirror/debian/cassandra.yml
+++ b/aptly/server/mirror/debian/cassandra.yml
@@ -1,4 +1,6 @@
 parameters:
+  _param:
+    apt_mk_version: stable
   aptly:
     server:
       mirror:
@@ -14,5 +16,5 @@
           publisher:
             component: extra
             distributions:
-              - trusty/nightly
-              - xenial/nightly
+              - ubuntu-trusty/${_param:apt_mk_version}
+              - ubuntu-xenial/${_param:apt_mk_version}