Fix silently failing Overseer Election joinAtHead during testDesignatedOverseerRestarts

OverseerRolesTest.testDesignatedOverseerRestarts has been failing consistently (around 2.5% of the time). I think this is because LeaderElection.joinElection does not respect the joinAtHead flag, if connectionIssues happen while setting the leader election nodes.

LeaderElection does not use the automatic retryOnConnLoss flags when doing zk operations. Instead, it waits for an error to come back, and it handles the retry itself. This is fine for the normal case, because it checks if node is represented in the leaderElection child nodes, and if so it ignores the connection loss. However when doing joinAtHead, if the childNode exists, but isn't at the place it should be, then the manual retry should be exercised.