rolling_update: unmask monitor service after a failure

if for some reason the playbook fails after the service was
stopped, disabled and masked and before it got restarted, enabled and
unmasked, the playbook leaves the service masked and which can make users
confused and forces them to unmask the unit manually.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1917680



Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>