engine: Correct remove-brick flow

Stop remove-brick and commit remove brick should not be
monitored as separate jobs.
Once migration of data is completed for remove-brick
the step status is updated.
Corrected parameters passed to commit brick and
the step message that is updated.
Refactored the endStepJob method.

Change-Id: I8f7a99f8ae87186c42e7346508b6abf0a2d1b7be
Signed-off-by: Sahina Bose <sabose@redhat.com>