This test shuts down two out of three managers and then asserts that the
swarm has a leader. A swarm that lost quorum won't necessarily have a
leader, and in this case only has one because the old leader is still
around. Soon SwarmKit will be changed so the leader gives up leadership
when quorum is lost. This will avoid confusing situations, like
read-only APIs succeeding, while ones that write to Raft hang.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>