Fix Service TTY test so signal handlers work

Noticed this test container not exiting correctly while debugging
another issue. Before this change, signals were being eaten by bash, now
they are hanlded by top. This cuts the test time in half since it
doesn't have to wait for docker to SIGKILL it.

Old:
PASS: docker_cli_swarm_test.go:840: DockerSwarmSuite.TestSwarmServiceTTY	18.997s

New:
PASS: docker_cli_swarm_test.go:840: DockerSwarmSuite.TestSwarmServiceTTY	6.293s

Signed-off-by: Brian Goff <cpuguy83@gmail.com>
(cherry picked from commit e6c5563ae9d083cc72fcf84796c542047ac0a299)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
Upstream-commit: eba485a3c6ac41bf844279c13a88844a512de4ac
Component: engine

Author: cpuguy83

components/engine/integration-cli/docker_cli_swarm_test.go

@@ -842,7 +842,7 @@ func (s *DockerSwarmSuite) TestSwarmServiceTTY(c *check.C) {
 
 	name := "top"
 
-	ttyCheck := "if [ -t 0 ]; then echo TTY > /status && top; else echo none > /status && top; fi"
+	ttyCheck := "if [ -t 0 ]; then echo TTY > /status; else echo none > /status; fi; exec top"
 
 	// Without --tty
 	expectedOutput := "none"