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 commit 'refs/changes/83/86083/2' of git://gold.eng.hq.xensource.com/git/github/openstack-dev/devstack into tmpbranch
Merge commit 'refs/changes/18/112018/1' of git://gold.eng.hq.xensource.com/git/github/openstack-dev/devstack into tmpbranch