Fix network names when using network isolation

Fix network names when using network isolation

When we merged If3989f24f077738845d2edbee405bd9198e7b7db we correctly
used name_lower for most things but we left out the the
OS::TripleO::Network resource which would cause errors like the
following:

Could not fetch contents for file:///tmp/tripleoclient-LdqQGJ/tripleo-heat-templates/network/internalapi.yaml

The reason is that the network filename is called internal_api.yaml.

Change-Id: I40f268668ed948e5d41ed0ff5a8fc954cef7b17c
Closes-Bug: #1697883