Add existing Keystone roles to gather/site.yml

Keystone roles where added previously but
where never added to the list of roles to
execute in the playbook. This corrects
that oversite.

Change-Id: I8dde374ad62563bc419b039cf6478be6fe6590d0

ansible/gather/site.yml

@@ -11,6 +11,7 @@
   roles:
     - nova
     - neutron
+    - keystone
     - mysql
     - rabbitmq