Add integration tests for existing allow/deny/net_out functionality

 - Add 'netdog' executable to test udp
 - Remove subnet_sharing_test as it's now subsumed by these tests

[#81502222]
Use REJECT rather than DROP for deny rules

Start of compatibility with security group changes in Warden. Also
start to pull out management of iptables rules in to golang wrappers.

[#82554176]