ansible.cfg: Fix SSH pipelining config

The docs aren't very clear on this to be honest, but this option
needs to be under a [ssh_connection] section.

See: http://docs.ansible.com/ansible/intro_configuration.html#pipelining
See: https://review.openstack.org/#/c/331499/

ansible.cfg

@@ -1,3 +1,5 @@
 [defaults]
-pipelining=True
 retry_files_enabled=False
+
+[ssh_connection]
+pipelining=True