[20.10 backport] Fix swarm rollback exitcode, and fix skipping verify step

[thaJeztah]

backport of #2939

fixes moby/moby#35312 'docker service rollback' returns error code 1

Two patches in this PR:

docker service rollback: fix non-zero exit code in some cases

Before this change:

$ docker service create --replicas=1 --name foo -p 8080:80 nginx:alpine
t33qvykv8y0zbz266rxynsbo3
overall progress: 1 out of 1 tasks
1/1: running   [==================================================>]
verify: Service converged

$ echo $?
0

$ docker service update --replicas=5 foo
foo
overall progress: 5 out of 5 tasks
1/5: running   [==================================================>]
2/5: running   [==================================================>]
3/5: running   [==================================================>]
4/5: running   [==================================================>]
5/5: running   [==================================================>]
verify: Service converged

$ echo $?
0

$ docker service rollback foo
foo
rollback: manually requested rollback
overall progress: rolling back update: 1 out of 1 tasks
1/1: running   [>                                                  ]
verify: Service converged

$ echo $?
0

$ docker service rollback foo
foo
service rolled back: rollback completed

$ echo $?
1

After this change:

$ docker service create --replicas=1 --name foo -p 8080:80 nginx:alpine
t33qvykv8y0zbz266rxynsbo3
overall progress: 1 out of 1 tasks
1/1: running   [==================================================>]
verify: Service converged

$ echo $?
0

$ docker service update --replicas=5 foo
foo
overall progress: 5 out of 5 tasks
1/5: running   [==================================================>]
2/5: running   [==================================================>]
3/5: running   [==================================================>]
4/5: running   [==================================================>]
5/5: running   [==================================================>]
verify: Waiting 1 seconds to verify that tasks are stable...

$ echo $?
0

$ docker service rollback foo
foo
rollback: manually requested rollback
overall progress: rolling back update: 1 out of 1 tasks
1/1: running   [>                                                  ]
verify: Service converged

$ echo $?
0

$ docker service rollback foo
foo
service rolled back: rollback completed

$ echo $?
0

$ docker service ps foo
ID             NAME      IMAGE          NODE             DESIRED STATE   CURRENT STATE           ERROR     PORTS
4dt4ms4c5qfb   foo.1     nginx:alpine   docker-desktop   Running         Running 2 minutes ago

Remaining issues with reconciliation

Note that both before, and after this change, the command sometimes terminates
early, and does not wait for the service to reconcile; this is most apparent
when rolling back is scaling up (so more tasks are deployed);

$ docker service rollback foo
foo
service rolled back: rollback completed

$ docker service rollback foo
foo
rollback: manually requested rollback
overall progress: rolling back update: 1 out of 5 tasks
1/5: pending   [=================================>                 ]
2/5: running   [>                                                  ]
3/5: pending   [=================================>                 ]
4/5: pending   [=================================>                 ]
5/5: pending   [=================================>                 ]
service rolled back: rollback completed

service rollback: always verify state

Prior to this change, progressbars would sometimes be hidden, and the function
would return early. In addition, the direction of the progressbars would sometimes
be "incrementing" (similar to "docker service update"), and sometimes be "decrementing"
(to indicate a "rollback" is being performed).

This fix makes sure that we always proceed with the "verifying" step, and now
prints a message after the verifying stage was completed;

$ docker service rollback foo
foo
overall progress: rolling back update: 5 out of 5 tasks
1/5: running   [>                                                  ]
2/5: starting  [===========>                                       ]
3/5: starting  [===========>                                       ]
4/5: running   [>                                                  ]
5/5: running   [>                                                  ]
verify: Service converged
rollback: rollback completed

$ docker service rollback foo
foo
overall progress: rolling back update: 1 out of 1 tasks
1/1: running   [>                                                  ]
verify: Service converged
rollback: rollback completed

- Description for the changelog

- Fix `docker service rollback` returning a non-zero exit code in some situations
- Fix inconsistent progress-bar direction on `docker service rollback`

[silvin-lubecki]

LGTM