Make the certificates role just run on deploy node

when run command "kolla-ansible -i multinode certificates",
the certificates file will generated in all nodes, it is
unnecessary, this ps to make it in deploy node.

Change-Id: I3e98ab498eeec3e6b8f170dd29c95f7ff9dbd6c0

ansible/certificates.yml

 ---
 - name: Apply role certificates
-  hosts: all
+  hosts: localhost
   roles:
     - certificates