Handle single default v4 gateway

If we are given a single v4 gateway, make sure we use it.
Otherwise assume the current gateway is OK, and do not
look at the gateways of the v4 subnets we are given.

Change-Id: Id85bda590367bf6772e2ae451e44bb0608987e26
Merge pull request #39 from dhellmann/bug/dhc-2072

Handle single default v4 gateway