Properly fail precheck for internal VIP's network

When kolla_internal_vip_address didn't belong to the same network as
network_interface, the prechecks would spit out an ugly error:

  Unexpected templating type error occurred on [snip]:
  argument of type 'NoneType' is not iterable"

This commit fixes the check to execute properly.

Change-Id: I24a066c212c78e105a923525b131724736ac74b6