tests: deflake TestAutopilot_RollingUpdate

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.

nomad/autopilot_test.go

@@ -210,7 +210,7 @@ func TestAutopilot_RollingUpdate(t *testing.T) {
 	retry.Run(t, func(r *retry.R) {
 		r.Check(wantRaft(servers))
 		for _, s := range servers {
-			r.Check(wantPeers(s, 3))
+			r.Check(wantPeers(s, 4))
 		}
 	})