Require use of mesh gateways in order for service mesh data plane
traffic to flow between peers.
This also adds plumbing for envoy integration tests involving peers, and
one starter peering test.
* Allow setting the mesh gateway mode for an upstream in config files
* Add envoy integration test for mesh gateways
This necessitated many supporting changes in most of the other test cases.
Add remote mode mesh gateways integration test
Ronald
R.B. Boyer
freddygv
Matt Keeler