nova.conf: fix incorrect neutron credentials.

Nova should use Neutron credentials to interact with Neutron.

Backport: liberty
Closes-Bug: #1512070
Change-Id: I78ec4c032d457b519ccfed9cab190afd65027048

ansible/roles/nova/templates/nova.conf.j2

@@ -102,7 +102,7 @@ auth_plugin = password
 project_domain_name = default
 user_domain_id = default
 project_name = service
-username = nova
+username = neutron
 password = {{ neutron_keystone_password }}
 
 [database]