integration-cli: add more debugging for TestSwarmClusterRotateUnlockKey

This test was updated in b79adac339173bf8bb6de6d0a061a97973c4b62b, but is still flaky;

```
20:24:13  FAIL: docker_cli_swarm_test.go:1333: DockerSwarmSuite.TestSwarmClusterRotateUnlockKey
20:24:13
20:24:13  Creating a new daemon at: /go/src/github.com/docker/docker/bundles/test-integration/3/DockerSwarmSuite.TestSwarmClusterRotateUnlockKey
20:24:13  [d6f95e679cb65] waiting for daemon to start
20:24:13  [d6f95e679cb65] waiting for daemon to start
20:24:13  [d6f95e679cb65] daemon started
20:24:13
20:24:13  Creating a new daemon at: /go/src/github.com/docker/docker/bundles/test-integration/3/DockerSwarmSuite.TestSwarmClusterRotateUnlockKey
20:24:13  [d204a02ba4780] waiting for daemon to start
20:24:13  [d204a02ba4780] waiting for daemon to start
20:24:13  [d204a02ba4780] daemon started
20:24:13
20:24:13  [d204a02ba4780] joining swarm manager [d6f95e679cb65]@0.0.0.0:2477, swarm listen addr 0.0.0.0:2478
20:24:13  Creating a new daemon at: /go/src/github.com/docker/docker/bundles/test-integration/3/DockerSwarmSuite.TestSwarmClusterRotateUnlockKey
20:24:13  [d873d6a842829] waiting for daemon to start
20:24:13  [d873d6a842829] waiting for daemon to start
20:24:13  [d873d6a842829] daemon started
20:24:13
20:24:13  [d873d6a842829] joining swarm manager [d6f95e679cb65]@0.0.0.0:2477, swarm listen addr 0.0.0.0:2479
20:24:13  [d204a02ba4780] Stopping daemon
20:24:13  [d204a02ba4780] exiting daemon
20:24:13  [d204a02ba4780] Daemon stopped
20:24:13  [d204a02ba4780] waiting for daemon to start
20:24:13  [d204a02ba4780] waiting for daemon to start
20:24:13  [d204a02ba4780] daemon started
20:24:13
20:24:13  [d873d6a842829] Stopping daemon
20:24:13  [d873d6a842829] exiting daemon
20:24:13  [d873d6a842829] Daemon stopped
20:24:13  [d873d6a842829] waiting for daemon to start
20:24:13  [d873d6a842829] waiting for daemon to start
20:24:13  [d873d6a842829] daemon started
20:24:13
20:24:13  docker_cli_swarm_test.go:1413:
20:24:13      c.Assert(err, checker.IsNil, check.Commentf("%s", outs))
20:24:13  ... value *exec.ExitError = &exec.ExitError{ProcessState:(*os.ProcessState)(0xc000934240), Stderr:[]uint8(nil)} ("exit status 1")
20:24:13  ... Error response from daemon: rpc error: code = Unknown desc = The swarm does not have a leader. It's possible that too few managers are online. Make sure more than half of the managers are online.
20:24:13
20:24:13
20:24:13  [d6f95e679cb65] Stopping daemon
20:24:13  [d6f95e679cb65] exiting daemon
20:24:13  [d6f95e679cb65] Daemon stopped
20:24:13  [d204a02ba4780] Stopping daemon
20:24:13  [d204a02ba4780] exiting daemon
20:24:13  [d204a02ba4780] Daemon stopped
20:24:13  [d873d6a842829] Stopping daemon
20:24:13  [d873d6a842829] exiting daemon
20:24:13  [d873d6a842829] Daemon stopped
```

The interesting bit there is that the retry loop should have a 3 second sleep before retrying,
but looking at the failure above, the test started (and failed) within a second, which means that
a different error / output was returned.

This patch adds some additional debugging to that test to see if we can catch the reason
this test is still flaky.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

integration-cli/docker_cli_swarm_test.go

@@ -1381,11 +1381,14 @@ func (s *DockerSwarmSuite) TestSwarmClusterRotateUnlockKey(c *testing.T) {
 				if err != nil && retry < 5 {
 					if strings.Contains(outs, "swarm does not have a leader") {
 						retry++
+						c.Logf("[%s] got 'swarm does not have a leader'. retrying (attempt %d/5)", d.ID(), retry)
 						time.Sleep(3 * time.Second)
 						continue
+					} else {
+						c.Logf("[%s] gave error: '%v'. retrying (attempt %d/5): %s", d.ID(), err, retry, outs)
 					}
 				}
-				assert.Assert(c, err == nil, outs)
+				assert.NilError(c, err, "[%s] failed after %d retries: %v (%s)", d.ID(), retry, err, outs)
 				assert.Assert(c, !strings.Contains(outs, "Swarm is encrypted and needs to be unlocked"))
 				break
 			}