Pin python-pymysql latest version MOS repo.

Change-Id: Ia42b38284331484df7bafe80773b7f5e1b28fd01
diff --git a/heat/server/cluster.yml b/heat/server/cluster.yml
index ee5df33..8e38f4f 100644
--- a/heat/server/cluster.yml
+++ b/heat/server/cluster.yml
@@ -6,6 +6,12 @@
 parameters:
   _param:
     cluster_public_protocol: https
+  linux:
+    system:
+      package:
+        python-pymysql:
+          fromrepo: ${_param:openstack_version}
+          version: latest
   heat:
     server:
       stack_domain_admin: