Add basic multiple backend gateway tests - negative functionality

[nephomaniac]

Add multiple backend gateway tests which validate the system during negative tests.

Negative:
Up/down test of GWs repeated at random intervals during packet streaming.
Repeated adding removing of GWs during packet streaming
Test GW teardown and recovery during high VM count (ie 500+ VMs) and basic/small packet test(ping?)
Long term burn in of multiple GWs during VM churn tests