Send the router its default gateway IP

Each router may have multiple public IPs, but we only want it to
use a single default gateway. We work out what that value should
be in the rug, and send it to the appliance so the appliance
does not have to figure it out for itself.

Change-Id: Ie8ae8a866ee46ed9b0b3d5ef79c2a4fadf1b7f0a
make it clear the new behavior is only for v4

Change-Id: Ia040c367121751ad5f50d76fedb77e30d9c0a238
Add tests for get_default_v4_gateway

Change-Id: I5366a773833f2f1029b4936e1cbfe294767c77f3
one more test case for ipv4 gateway

Change-Id: I6aa63566ddb6a0a25c724a727efb0f4470475e22
fix pep8 issues

Change-Id: If042eded3f98f17b087047c1707e20f68e495603
Merge pull request #143 from dhellmann/bug/dhc-2072

Send the router its default gateway IP