tenant_network_type files are all OVS related

[Ian Wells]

A tenant_network_type should be selected for each node, but as it happens this only has any great meaning when the OVS plugin is selected as (a) the tenant types are all OVS specific and (b) the settings within them are all OVS specific. If one were to use a controller without a tenant network specification then this would make no sense; nor does it make sense for controllers with multiple tenant network specifications such as ml2, and even if it did the settings don't affect other plugins at all. Not obvious to me that this should have a hiera_data directory of its own.