Fix TestSwarmClusterRotateUnlockKey

TestSwarmClusterRotateUnlockKey had been identified as a flaky test. It
turns out that the test code was wrong: where we should have been
checking the string output of a command, we were instead checking the
value of the error. This means that the error case we were expecting was
not being matched, and the test was failing when it should have just
retried.

Signed-off-by: Drew Erny <drew.erny@docker.com>
(cherry picked from commit b79adac)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Author: dperny

integration-cli/docker_cli_swarm_test.go

@@ -1308,7 +1308,7 @@ func (s *DockerSwarmSuite) TestSwarmRotateUnlockKey(c *check.C) {
 			// an issue sometimes prevents leader to be available right away
 			outs, err = d.Cmd("node", "ls")
 			if err != nil && retry < 5 {
-				if strings.Contains(err.Error(), "swarm does not have a leader") {
+				if strings.Contains(outs, "swarm does not have a leader") {
 					retry++
 					time.Sleep(3 * time.Second)
 					continue
@@ -1400,7 +1400,7 @@ func (s *DockerSwarmSuite) TestSwarmClusterRotateUnlockKey(c *check.C) {
 				// an issue sometimes prevents leader to be available right away
 				outs, err = d.Cmd("node", "ls")
 				if err != nil && retry < 5 {
-					if strings.Contains(err.Error(), "swarm does not have a leader") {
+					if strings.Contains(outs, "swarm does not have a leader") {
 						retry++
 						time.Sleep(3 * time.Second)
 						continue