Pin python-pymysql latest version MOS repo.

Change-Id: Ia42b38284331484df7bafe80773b7f5e1b28fd01
diff --git a/keystone/server/single.yml b/keystone/server/single.yml
index 72dc045..c35ef73 100644
--- a/keystone/server/single.yml
+++ b/keystone/server/single.yml
@@ -7,6 +7,12 @@
     mysql_admin_user: root
     mysql_admin_password: password
     mysql_keystone_password: password
+  linux:
+    system:
+      package:
+        python-pymysql:
+          fromrepo: ${_param:openstack_version}
+          version: latest
   keystone:
     server:
       enabled: true