Mock dogpile cache in API tests

Change-Id: I5e168e5aeb9b3d189c85dda36ee84a1183623af7
Support empty gateway_ip field on subnets

If the user doesn't want to route out of a subnet, let them leave the
gateway IP blank.

Change-Id: Ibc53b47cf788bbb4f4e48297a0c1c692f16b81d8
Fix flake8

Change-Id: Ie5a69e403a8bac0c27d796cbf97875e3df10ba19
Merge pull request #58 from dhellmann/handle-missing-gateway

Support empty gateway_ip field on subnets