In conformance tests, deploy only the backends actually needed by the tests

[mlavacca]

What would you like to be added:

As of now, when running the conformance tests, all the backends are deployed upfront (HTTP, gRPC, TLS), even if they won't be used by the actual tests. We should deploy only the backends actually used by the tests. We can easily figure out which backends are going to be used by looking into the activated features.

/area conformance