Do not force slack package in the defaults.
diff --git a/metadata/service/master/single.yml b/metadata/service/master/single.yml
index eb91c01..0d27f12 100644
--- a/metadata/service/master/single.yml
+++ b/metadata/service/master/single.yml
@@ -12,6 +12,8 @@
         network: any
         port: 8080
         protocol: http
+      pkgs:
+        - jenkins-jobs-slack
       plugins:
       - name: git
       - name: metadata