[test] fix legacy default

[asraa]

Signed-off-by: Asra Ali asraa@google.com

Commit Message: Because flags are default false, if use_new_codecs_in_integration_test flag wasn't explicitly set, it would be false. Swapped so that the flag refers to legacy codecs (used in compile time options), and by default, we use new codecs unless the flag is set FOR legacy.
Risk Level: Low
Testing: Tested integration tests.

Before on RouterDownstreamDisconnectBeforeResponseComplete:

[2020-11-19 15:03:04.236][20][trace][http] [source/common/http/http1/codec_impl_legacy.cc:465] [C3] completed header: key=host value=host
[2020-11-19 15:03:04.236][20][trace][http] [source/common/http/http1/codec_impl_legacy.cc:465] [C3] completed header: key=x-forwarded-proto value=http
[2020-11-19 15:03:04.236][20][trace][http] [source/common/http/http1/codec_impl_legacy.cc:465] [C3] completed header: key=x-request-id value=ccb817e4-b6c0-473b-bd5f-6e04e1f425c0
[2020-11-19 15:03:04.236][20][trace][http] [source/common/http/http1/codec_impl_legacy.cc:465] [C3] completed header: key=x-envoy-expected-rq-timeout-ms value=15000

After

[2020-11-19 14:56:33.979][20][trace][http] [source/common/http/http1/codec_impl.cc:489] [C3] completed header: key=host value=host
[2020-11-19 14:56:33.979][20][trace][http] [source/common/http/http1/codec_impl.cc:489] [C3] completed header: key=x-forwarded-proto value=http
[2020-11-19 14:56:33.979][20][trace][http] [source/common/http/http1/codec_impl.cc:489] [C3] completed header: key=x-request-id value=bbd41b99-2689-4470-b407-2f516cda4a1c
[2020-11-19 14:56:33.979][20][trace][http] [source/common/http/http1/codec_impl.cc:489] [C3] completed header: key=x-envoy-expected-rq-timeout-ms value=15000

If there's a way to make a config setting with a default value, let me know.

[alyssawilk]

Nice catch!