Add basic multiple backend gateway tests - functionality

[nephomaniac]

Add basic multiple backend gateway tests.
These tests should validate usage of > 1 backend gateways.
Suggested tests/tasks for both BGP and Static modes may include...

Tests:
Basic routing test, ie do packets route over each GW
Test how GWs behave with a GW torn down during packet streaming
Test how GWs behave with a GW recovered/repaired during packet streaming
Test how GWs behave with a new GW added and brought online during packet streaming
Test how GWs behave with an existing GW removed from config during packet streaming
Max # of GWs supported