Change hardcoded nightly repo to variable

Change-Id: Ifbe7369ddc2937f8abd406eb7097a67d5373c863
diff --git a/aptly/server/mirror/ubuntu/xenial/influxdb.yml b/aptly/server/mirror/ubuntu/xenial/influxdb.yml
index 87930b9..a2ac597 100644
--- a/aptly/server/mirror/ubuntu/xenial/influxdb.yml
+++ b/aptly/server/mirror/ubuntu/xenial/influxdb.yml
@@ -1,4 +1,6 @@
 parameters:
+  _param:
+    apt_mk_version: stable
   aptly:
     server:
       mirror:
@@ -12,4 +14,4 @@
           publisher:
             component: influxdb
             distributions:
-              - ubuntu-xenial/nightly
+              - ubuntu-xenial/${_param:apt_mk_version}