XenApi: Fix domu typo in neutron setup

A typo has been introduced during a previous refactor, leaving us with
two neutron config files, one with domu and one with domU extension.
This fix corrects the mistake, thus always the .domU file is used.

Change-Id: Iffb6e2f70915f50ee590c375494c6f642412b543
Merge "XenApi: Fix domu typo in neutron setup"
Merge branch 'master' of github.com:openstack-dev/devstack