Improve ZkFailoverTest

ZkFailoverTest simulates a failed restart where Solr does not come up clean and then restart Zookeeper. It does this by setting system property waitForZk=1.

Now after SOLR-16975, NodeConfig will no longer load solr.xml from zookeeper, so we now keep a small piece of code doing a dummy connection to Zookeeper just to fail / throw exception if it times out. This is dirty, so we should clean up the test logic:

• Remove zk connection code from NodeConfig
• Re-write ZkFailoverTest to simulate this in a different way

Or maybe we could even remove ZkFailoverTest, I don't see what this corner case is really testing. Probably it was some kind of issue in the past that we fixed and wanted test coverage for?