BZ #1154785: Ensure Public API subnet has a default gateway

https://bugzilla.redhat.com/show_bug.cgi?id=1154785

The public API subnet must have a default gw set. This ensures that
the public API type cannot be assigned to a subnet with the defgw.
Merge pull request #358 from bcrochet/bz/1154785

BZ #1154785: Ensure Public API subnet has a default gateway