packaging: changed tasks polling in engine-upgrade

Modified checkRunningTasks: during engine upgrade, sample the current
situation of asynchronous tasks and compensations and continue the
upgrade, exiting the check loop one second after there are no async
tasks or compensations left.
Currently the stopping of asynchronous tasks and compensations
could take 3 minutes so every ~3 minutes
(MAINTENANCE_TASKS_WAIT_PERIOD) if there are async tasks still around,
allows the user to retry or abort.

Other than that it doesn't change the behavior of the script.

Removed MAINTENANCE_TASKS_CYCLES since it's not used anymore.

Fixed a small typo in MSG_ERROR_FAILED_CONVERT_ENGINE_KEY.

Change-Id: I32eaeb2427fcc9c4566ca12bf547d49a308a8937
Bug-Url: https://bugzilla.redhat.com/877749
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>