I hypothesize that the flakiness in rolling update is due to shutting
down s3 server before s4 is properly added as a voter.
The chain of the flakiness is as follows:
1. Bootstrap with s1, s2, s3
2. Add s4
3. Wait for servers to register with 3 voting peers
* But we already have 3 voters (s1, s2, and s3)
* s4 is added as a non-voter in Raft v3 and must wait until autopilot promots it
4. Test proceeds without s4 being a voter
5. s3 shutdown
6. cluster changes stall due to leader election and too many pending configuration
changes (e.g. removing s3 from raft, promoting s4).
Here, I have the test wait until s4 is marked as a voter before shutting
down s3, so we don't have too many configuration changes at once.
In https://circleci.com/gh/hashicorp/nomad/57092, I noticed the
following events:
```
TestAutopilot_RollingUpdate: autopilot_test.go:204: adding server s4
TestAutopilot_RollingUpdate: testlog.go:34: 2020-04-03T20:08:19.789Z [INFO] nomad/serf.go:60: nomad: adding server: server="nomad-137.global (Addr: 127.0.0.1:9177) (DC: dc1)"
TestAutopilot_RollingUpdate: testlog.go:34: 2020-04-03T20:08:19.789Z [INFO] raft/raft.go:1018: nomad.raft: updating configuration: command=AddNonvoter server-id=c54b5bf4-1159-34f6-032d-56aefeb08425 server-addr=127.0.0.1:9177 servers="[{Suffrage:Voter ID:df01ba65-d1b2-17a9-f792-a4459b3a7c09 Address:127.0.0.1:9171} {Suffrage:Voter ID:c3337778-811e-2675-87f5-006309888387 Address:127.0.0.1:9173} {Suffrage:Voter ID:186d5e15-c473-e2b3-b5a4-3259a84e10ef Address:127.0.0.1:9169} {Suffrage:Nonvoter ID:c54b5bf4-1159-34f6-032d-56aefeb08425 Address:127.0.0.1:9177}]"
TestAutopilot_RollingUpdate: autopilot_test.go:218: shutting down server s3
TestAutopilot_RollingUpdate: testlog.go:34: 2020-04-03T20:08:19.797Z [INFO] raft/replication.go:456: nomad.raft: aborting pipeline replication: peer="{Nonvoter c54b5bf4-1159-34f6-032d-56aefeb08425 127.0.0.1:9177}"
TestAutopilot_RollingUpdate: autopilot_test.go:235: waiting for s4 to stabalize and be promoted
TestAutopilot_RollingUpdate: testlog.go:34: 2020-04-03T20:08:19.975Z [ERROR] raft/raft.go:1656: nomad.raft: failed to make requestVote RPC: target="{Voter c3337778-811e-2675-87f5-006309888387 127.0.0.1:9173}" error="dial tcp 127.0.0.1:9173: connect: connection refused"
TestAutopilot_RollingUpdate: retry.go:121: autopilot_test.go:241: don't want "c3337778-811e-2675-87f5-006309888387"
autopilot_test.go:241: didn't find map[c54b5bf4-1159-34f6-032d-56aefeb08425:true] in []raft.ServerID{"df01ba65-d1b2-17a9-f792-a4459b3a7c09", "186d5e15-c473-e2b3-b5a4-3259a84e10ef"}
```
Note how s3, c3337778, is present in the peers list in the final
failure, but s4, c54b5bf4, is added as a Nonvoter and isn't present in
the final peers list.
816a93ed4a