Parameterize echo listener address

[hennna]

This change corresponds to changes in envoyproxy/envoy#883 to allow for Ipv6 parameterized integration testing. In envoyproxy/envoy#883, we SetUp the test_server for each test rather than once in SetUpTestCase.

[htuch]

LGTM, but we should also ideally update the submodule hash as well. Some rambling about CI below for further discussion..

I think we have CI coverage of this repository via the main envoy Travis (when we bump its hash for this repo). But, we might break stuff for external folks who rely on this repo as an example without CI and use the submodule build path.

Of course, if we add this, we get into a chicken-and-egg problem, as we can't update either envoy.git or envoy-filter-example.git atomically. This makes my head hurt.

I think the best thing to do is accept temporary brokeness in this repo and ship the PR as is.

[htuch]

Actually, after thinking about this, I think we should merge this into a branch called envoy-ci. That way, envoy.git can depend on this for CI, without having the example we provide to developers on the master branch breaking.