interop-testing: overrideAuthority breaks JWT

Commit 65e4d9f4 broke the jwt_token_creds. It is believed to be because
the JWT does not see the authority passed to overrideAuthority. So the
changes to interop-testing client are temporarily reverted here. Note
that this breaks GRPC_PROXY_EXP testing, so the incompatibility needs to
be resolved.

Solving #2682 will allow reverting this change.

Fixes grpc/grpc#9497
Fixes #2680