Netty exclusion through ZooKeeper not effective as intended

Picking up where psomogyi has been digging this week. We've been seeing an issue where MiniDFS-based tests fail to start due to missing netty classes.

HBASE-25969 seems to have intended to remove transitive Netty but was ineffective (at least for hadoop.profile=3.0). The dependency exclusion was for io.netty:netty and io.netty:netty-all but ZooKeeper 3.5.7 transitively depends on netty-handler and netty-transport-native-epoll  (per https://search.maven.org/artifact/org.apache.zookeeper/zookeeper/3.5.7/jar)

The funny part is that we should have seen failures in any hbase unit test using MiniDFS because we excluded netty and netty-all in HBASE-25969, but because we missed the exclusions, we still keep running.

The intent of HBASE-25969 was good, but I think we need to revisit the execution. We need netty dependencies on the scope=test classpath. We just want to keep them off the scope=compile classpath (out of our client and server jars).

disclaimer: I have not yet looked at 2.5.x or master yet to see if this also affects them.