This is a follow-up to SERVER-25270.

1. The concurrent fuzzing tasks should be added to the "Linux" variant
2. The concurrent fuzzing tasks should be added to the "Ubuntu 14.04 (RocksDB)" variant
3. All fuzzing tasks should be removed from the "~ Enterprise RHEL 6.2 C++14 DEBUG" variant because the fuzzer can't run against DEBUG builds (SERVER-25567 is a related issue)
4. The fuzzer task list should be expanded or removed from the "z ASIO MessagingPort SSL Ubuntu 1604 64-bit" variant