Fix pinning of mk repositories

Change-Id: I5b9c0af413f0e9b96662e2168e01519c27fa14fd
diff --git a/linux/system/repo/mcp/extra.yml b/linux/system/repo/mcp/extra.yml
index 49e4532..00de9ea 100644
--- a/linux/system/repo/mcp/extra.yml
+++ b/linux/system/repo/mcp/extra.yml
@@ -10,6 +10,6 @@
           key_url: "http://apt-mk.mirantis.com/public.gpg"
           clean_file: true
           pin:
-          - pin: 'release a=${_param:openstack_version}-holdback'
+          - pin: 'release a=${_param:apt_mk_version}'
             priority: 1100
             package: '*'