SOLR-6428: Occasional OverseerTest#testOverseerFailure fail due to missing election node.
SOLR-5596: OverseerTest.testOverseerFailure - leader node already exists.

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1620320 13f79535-47bb-0310-9956-ffa450edef68