Handle failed neutron-plugin-ml2.yaml lookup
Handle failed neutron-plugin-ml2.yaml lookup

This handles the puppet/services/neutron-plugin-ml2.yaml template
GET failure by simply hard coding the 'ovn' default which matches
the latest t-h-t templates default anyway for this template.

Change-Id: Ic7c1817671467f4ec2ef823f52e165c2c1f230ca

openstack/tripleo-common,openstack/tripleo-common