Move away from using hidden_envoy_deprecated_hosts

[oschaaf]

In #376 I had to make some changes in tests, to avoid asserts being triggered by deprecations in envoyproxy/envoy#9663.

This reminded me, Nighthawk is still using hidden_envoy_deprecated_hosts in some places.
It is probably a good idea to stop doing that.

[oschaaf]

While getting this to work, I noticed that there's some duplication going on for the cluster setup code.
I filed #387 for that.